What exacly is the so-called "formless" jhana?

I am so sorry for the long answer. It's required all of this knowledge to understand the meditation's idea.

What is jhāna?

Jhāna (concentration meditation) is the meditation method to cease the desire (chanda-rāga) of the world (loka) by changing mind's attention to the opposite object of the world.

What is the world?

Living being (satta) is the world (loka), 5 clinging-aggregates (upādāna-khandha[1][2]).

Loka means breakable thing. And what we can notice 3 characteristics must be break, breakable. What is unstable (anicca), stressful (dukkha), and non-self (anattā) is what we can notice it's instability-characteristic (anicca-lakkhaṇa), e.g. it's arising and vanishing, stress-characteristic, e.g. it's stressing to arise and to vanish in every millisecond, and uncontrollable characteristic, e.g. it's variant variables.

Each aggregate has 3 characteristics, so every aggregate (pañca-khandha) is breakable, and it is called Loka. There are 2 kinds of breakable aggregates, lokiya (with clinging) and lokuttara (no clinging). However the practitioner should comprehend only clinging aggregates (upādāna-khandha) to meditate insight meditation, Buddha called clinging aggregates (upādāna-khandha), in first noble truth, not only aggregates (khandha).

The genius people can notice 3 characteristics of every living being, so they try to cease all possible cases of living beings' rebirth by the meditation.

There are many method of the meditation of those genius people which doing the difference methods and each method causes the difference results, some is not able to pause any world, some can pause 5 string world (kāma loka), some can pause form concept world (rūpa loka), some can cease all worlds (kāma loka/rūpa loka/arūpa loka).

How many kinds of jhāna?

There are 3 kinds of jhāna:

1. Form jhāna (rūpa jhāna) - meditation of changing mind's attention form five strings world (kāma-loka) to form's concept world (rūpa-paññatti-loka) to disable 5 sense attachments arising (kāmachandarāga). This meditation's step, 1st-4th jhāna, pause attachment of five strings and mental factors which can drop the quality of concentration down to five strings world. The attained people are going to reborn in the world without five string attachment, but it still has forms, such as eyes to watch the form to meditate rūpa-jhāna.
2. Formless jhāna (arūpa jhāna) - meditation of changing mind's attention form form's concept world to formless world (arūpa-loka) to disable all form arising, which make rūpa jhāna&world still nearby five strings attachment. This meditation change the object to detach from mind and mental factors (arūpa-loka) as much as they can. However, it's useless, the number of mental factors still be the same, 30 factors, from the 1st formless jhāna to the 4th formless jhāna. The attained people are going to reborn in the world without form, but it still have mind and mental factors (nāma-khandha).
3. Three characteristic jhāna (lakkhaṇa-jhāna) - vipassanā; insight meditation. Meditation of changing mind's attention form all worlds to nibbāna (cessation of world). The attained magga people do not reborn any more in maximum 7 rebirths.

Jhāna bases vipassanā, especially for the practitioner who attach five strings strongly (taṇhā-carita).

How to disable 5 sense attachment?

Keeping the sixth sense mind on concept object (paññatti;paṭibhāga-nimitta) by practice to change the the sixth sense memory of the five strings (kāmaguṇa) to concept object (paññatti;paṭibhāga-nimitta) then meditate that practice to go on for a long period, a hour or a day.

How to disable all form arising?

Keeping the sixth sense mind on the object which has no any relation with form (arūpa) by practice to change the the sixth sense memory of form concept (rūpa-paññatti;paṭibhāga-nimitta) to formless object (arūpa) then meditate that practice to go on for a long period, a hour or a day.

What is the problem?

The jhāna practitioner still keep the self attachment of their meditation mind&mind factors as "it's mine. It's me, not the other. It's myself, so I can control it". This is the reason why we need Buddha.

See Ma.Mu. Cūḷasīhanādasutta, Mahādukkhakkhandhasutta, the path of purification in pathavīkasiṇa chapter and aruppā chapter, and this article.

The cessation of concious after all the jhana is exhausted and the life force still continue with its existing mind and body phenomenon. One cancelled this life binding contract upon cessation of mind and witnessing this concious phenomenon with the highest concious cultivated in the journey before the last stop of this world concious (human highest and most subtle concious after he return to continue to the final end of this life)upon the life expired the concious this time finally stop. (no more continue Concious to fuel life existing force) just like the candle lamp oil is exaushted no more flame will continue to burn.

The cessation of concious after all the jhana is exhausted and the life force still continue with its existing mind and body phenomenon. One cancelled this life binding contract upon cessation of mind and witnessing this concious phenomenon with the highest concious cultivated in the journey before the last stop of this world concious (human highest and most subtle concious after he return to continue to the final end of this life)upon the life expired the concious this time finally stop. (no more continue Concious to fuel life existing force) just like the candle lamp oil is exaushted no more flame will continue to burn. What has to be done is done and the and any attempt to recirculate about further human concious will bring about ignorance and confusion and not wanting to let go of this only bring one back to attachment and thus continue the subject of mind purification taking every listener into a merry go round cycle of ignorance. Hope practioner of mind purification be like the sky and allowing all kinds of cloud formation to take place without indifference as all compound things are subject to arise and decay. But this time we have withiness the decay of all concious in the mind purification process. (withiness while as a living being) BUDDHAM SARANAM GACCHAMI

The cessation of concious after all the jhana is exhausted and the life force will still continue with its existing mind and body phenomenon. One has withiness and cancelled this life binding contract of ignorance about reality of life and upon cessation of mind during the mind purification training brings one to direct awareness and witnessing this concious cessation phenomenon with the highest concious cultivated in this journey before the last stop of this world concious (human highest and most subtle concious will return to continue this life to final end)upon the life expired the concious this time finally stop(he return to claim the contract) No more continue Concious to fuel presant life existing force. Just like the candle lamp oil is exaushted no more flame will continue to burn. What has to be done is done and that any attempt to respeculate about further human concious will bring about ignorance and confusion and not wanting to let go of this only bring one back to ATTACHMENT(the root cause of all sufferring explain by the BUDDHA) and thus continue the subject of mind purification taking every listener into a merry go round cycle of ignorance. Hope practioner of mind purification be like the sky and allowing all kinds of cloud formation to take place without indifference as all compound things are subject to arise and decay. But this time we have withiness the decay of all concious in the mind purification process. (withiness while as a living being) BUDDHAM SARANAM GACCHAMI

With my subjective view and personal limitations, it is my opinion that these things can only be experienced. Its of no use to talk of them unless one has experienced them.

Namo Thassa Bhagavatho Arahatho Samma Sambudhdhassa.

According to my understanding from what I learned from Lord Buddha teachings, please correct me if I'm wrong,

Formless Dyana, can only be achieved after completing all the form Dyana. They can be only achieved by Kasina meditation. Lift mind to the 4th Dyana and spread the Kasina light to the infinite. Then with the thought of the disappointed nature of the form, meditate on the space of the Kasina light but not about the Kasina Nimitta. Then Kasina light will disappear and infinite space becomes the object of the meditation. At some point while meditation mind raise to the Dyana upon the infinite space. This Dyana referred as the Akasananchayatanaya. With this Dyana he's mind is completely taken away from senses. So no sound is heard... Etc. There are 3 more formless Dyana. It is said that Formless Dyana are more softer and calmer than form Dyana. If person doesn't lose Dyana he will born in one of the four formless Brahma world's according to their Dyana level. Formless Brahma levels are the highest lifespan starting from 16000 Maha kalpas.

According to my understanding, they are excluded since form Dyana are the best and sufficient Dyana for Vipassana meditation. Also the Dyana components of formless Dyana are same of 4th Form Dyana. They are Upekka, Ekaggata. Also target of achieving Dyana is to start Vipassana meditation. I believe Formless Dyana cannot be used to perform Vipassana for form(Rupa) but only on mind(Nama) since mind is detached from form.

Please note that it's not easy to even lift your mind to the first form Dyana. So even to think about formless Dyana one should poses all the form Dyana. Not possessing the form Dyana in other words rupawachara Dyana no one can lift mind to the formless level. So its useless if someone thinks it's easy and trying to lift mind to formless Dyana as his first step. It will not work that way.

You read "The dimension of nothingness" and probably, if you have the wit, you will say 'But that which is nothing is dimensionless, if it has dimension then it is something?"

Don't stop there ask "what is small?"

Scientists are hunting for the smallest particle, but whatever the smaller particle they may discover, again if you have a wit you will say " But if the smallest is measurable it can be divided then it is not the smallest" Right there, you are declaring the dimension of nothingness, but only in reverse.

Some enlightened beings like the Buddha see pass this hurdle of emptiness, but for the rest of us, it is a confusing loop and end in suffering.

Keep meditating you may win the prize!

Perhaps you have some help if you understand the 40 meditation object. Ther are four formless objects hence the rest 36 should be form objects. My understanding is very close to the understanding of Santa100 above.

https://en.wikipedia.org/wiki/Kamma%E1%B9%AD%E1%B9%ADh%C4%81na

AN9.41 provides a wonderfully detailed experience of renunciation into and through these dimensions.

Why don’t I, going totally beyond perceptions of form, with the ending of perceptions of impingement, not focusing on perceptions of diversity, aware that “space is infinite”, enter and remain in the dimension of infinite space?’

The sutta also deals with the doubts you've expressed and basically states that faith and a willingness to shift outlook and renounce is the key to understanding and experiencing each dimension. They're progressive.

I haven’t seen the drawbacks of forms, and so I haven’t cultivated that.

Which leads to the following encouragement:

Suppose that, seeing the drawbacks of forms, I was to cultivate that. And suppose that, realizing the benefits of the dimension of infinite space, I was to develop that. It’s possible that my mind would be eager for the dimension of infinite space; it would be confident, settled, and decided about it. And I would see it as peaceful

Although we could discuss infinite space as the absence of forms, it wouldn't compare to actually experiencing that infinite space.

Note: this is inference, not experience. The earlier stuff has worked in this manner for me and has given me confidence to just keep going...

To understand the exact difference between the 4 rupa jhanas, and the arupa samadhi, the short formula for the 5th attainment, the base of infinite space, contains the details. The formula is too terse to understand on its own, so this article provides the glosses from other suttas that explain the detailed meaning of the formula. http://lucid24.org/sted/8aam/8samadhi/smd5/index.html

In short, rupa are the 4 elements and 31 body parts, and in the context of 4 jhanas, rupa and kaya are referring to the same physical body. In the arupa samadhis, attainemnts #5-9, the mind faculty is divorced from the 5 senses - you can't feel the breath, you can't feel mosquito bites, you can't feel leg pain, you can't find your body spatially with kinasethetic sensation.

As AN 5.28 jhana similes clearly describe explicitly, in 4 jhanas one does have kinaesthetic sensation of the anatomical body.

Going off sutta and abhidhamma;

Suppose a person would see a sight with the eye. If the person was to close the eye and recall in mind the forms that were seen, he could do that.

When he first saw with the eye, the 'eye' was implicated in contact with visible forms (shapes&colors).

When recalling the sight, the eye wasn't implicated, the contact was only at mind base and with ideation which was of shapes&colors as can be seen with the 'eye', resultant perception is a different one from the ceased eye-consciousness but it was assosiated.

In arupa jhana the contact is also at the mind base and with ideation but the ideation has nothing to do with what is seen, heard, smelled, tasted or sensed with the body.

It is said that at that time intellect is devorced from the five faculties (senses derived from the body).

In the first example the object of recollection was tied to what can be seen. In the case of arupajhana one won't perceive forms & colors anymore nor feel with the body or imagine or recall what has to do with the world of form in principle.

If we imagine what the perception of infinite space is like, we can only visualize it in terms of darkness or some other color/shape; we can visualize nothingness no more than we can paint a music piece but we can perceive it with intellect by the stilling/fading of the other elements, form element in particular, arupa jhana.

Fading of form is pretty much the fading of senses derived from the body and any ideation that may arise of and in regards to the bodily senses.

Ie one is percepient of only space in the 1st arupa jhana.

Space unlike color&taste is of the mental sphere, is something we can't point to but we know it is there and can think about it, seeing it with intellect/discerning this space element when we think about what the senses present to make sense of what the senses present.

In this sense the space element can be seen with intellect, we know it plays part, comes into play, it is invisible but real.

In terms of game engine development; it as if an element or class of elements no longer get rendered into being, so there is then only the rendering of the remaining "physics", no rendering of environment or world. Maybe this can be a helpful analogy but in the dhamma that which renders is also the rendered, the system being talked about is closed, autonimously directing and self sustaining one.

Each arupa jhana is conveived, felt & perceived differently.

As to Right Concentration.

The entering into & remaining in this or that jhana is also called "a pleasant abiding".

And it's true that entering into & remaining in both rupa & arupajhana is called 'a pleasant abiding'; and only the entering into & remaining in rupa jhana is called Samma Samadhi (Right Concentration).

The Arupa Jhanas are probably not proclaimed to be a path constituent because they may be path for one person and not be the path for another.

In a sutta with Cunda(?) He askd whether those Arahants had magical powers or arupa jhanas and they being liberated by wisdom said no they didn't have the arupa jhanas, whereas other people do have these attainments not all arahts did; one can do without is the point.

Therefore it is probably not always true and is therefore not included in the universal expression of 8FNP.

Afaik Abhidhamma delineates the path constituent of right concentration for arupa jhanas.

Since they're "formless", they can't really be "exactly" described right? :-) Anyway, Ven. Bodhi's "Noble Eightfold Path" gave some idea about what they are. Also, while the four jhanas make up the usual textual definition of right concentration, that doesn't mean SammaSamadhi only involves just those 4 exclusively.

"Beyond the four jhanas lie the four immaterial states, levels of absorption in which the mind transcends even the subtlest perception of visualized images still sometimes persisting in the jhanas. The immaterial states are attained, not by refining mental factors as are the jhanas, but by refining objects, by replacing a relatively gross object with a subtler one. The four attainments are named after their respective objects: the base of infinite space, the base of infinite consciousness, the base of nothingness, and the base of neither-perception-nor-non-perception.[65] These states represent levels of concentration so subtle and remote as to elude clear verbal explanation. The last of the four stands at the apex of mental concentration; it is the absolute, maximum degree of unification possible for consciousness. But even so, these absorptions reached by the path of serenity meditation, as exalted as they are, still lack the wisdom of insight, and so are not yet sufficient for gaining deliverance."