Does Abhidhamma explain how Attention Shift happens

You might want to differentiate between Abhidhamma and Abhidhammamattha-sanghana which is non-canonical commentary from the 8-12th century.

I personally don't see how Abhidhamma posits existence of mind-moments and i find the notion entirely foolish because the semantic properties of 'a moment' are something that has a beginning, a middle & an end, whereas

what's called 'mind,' 'intellect,' or 'consciousness' by day and by night arises as one thing and ceases as another. Just as a monkey, swinging through a forest wilderness, grabs a branch. Letting go of it, it grabs another branch. Letting go of that, it grabs another one. Letting go of that, it grabs another one. In the same way, what's called 'mind,' 'intellect,' or 'consciousness' by day and by night arises as one thing and ceases as another.
SN 12.61

Talking about 'mind-moments' is akin to talking about 'now-moments', 'now' as in 'present[time]-moment'.

The concept of 'now' does not in it's semantic property assume any temporal duration, iow a 'now' doesn't last for some time before becoming a 'not-now'. It doesn't arise & cease as one thing.

The past, reverends, is one end. The future is the second end. The present is the middle. And craving is the seamstress …
AN 6.61

Talking about 'mind-moments' is akin to talking about 'now-moments', 'now' as in 'present[time]-moment'.

The concept of 'now' does not in it's semantic property assume any temporal duration, iow a 'now' doesn't last for some time before becoming a 'not-now'. It doesn't arise & cease as one thing.

The past, reverends, is one end. The future is the second end. The present is the middle. And craving is the seamstress …
AN 6.61

It's the nature of all formations to be not lasting, break apart, get sick, age, decay, good householder. Mind-factors are like that, objects of mind, like-a-like. Whether grasping another stand, or where, depends on ones gained inclination and related effort.

Generally proper/good association is food for right attention, first the association outwardly, then inwardly as well.

The Avijja Sutta (AN 10.61) would explain the relation of right attention and the other chains in regard of common wheel as well as path.

Sure, the window isn't large to grasp the way of attention blessed, yet possible right here.

In abhidhamma style: what determines what kind of moment of consciousness will arise, i.e. what the content of the next moment of consciousness will be.

There are two kind of attention shift. Voluntary and involuntary. In case of voluntary attention shift the consciousness had set the intention for that shift previously in any of recent mind moment. via a desire or resolve .Its called Vacī Sankhāra or internal speech . Which was preceded by vitaka and vicara on the object which was available in that mind moment. E.g when you sit for meditation you first verbally or non verbally set intention to restrain the scope of attention to nose. I will stay focused only at nose sensations. This is called paying attention.

In case of involuntary attention shift it is based on strong sense object received at 6 sense doors. which has considered crucial by the unconscious. and thrown immediately to the consciousness. e.g a loud sound forces the attention shift. or strong past memory pops up.

There are many complex logics by the unconscious. and one interesting as I think is attention span. Every human being has different attention span. the unconscious will will not allow any object to stay more then that. and via meditation practice one can increase this attention span. There are many other factors like loss of interest , or dullness of consciousness.

This unconscious activity is outside of consciousness. Buddhism just puts it under Sankhāra Khanada. however hinduism goes further and calls it a organ named manasa(conceit). Mahayan coined word called alay-Vijnan. As per abhidhamma manasa or vijnana are contaminated states of citta. The pure bhavanga vibrates when an object touches it. We can guess that manasa is place where ditthi, fase self or mara reside. even kilesa , moha or avijja is a near term to this unconscious behaviour.

Out of all sankhara ,or anusaya those which are common, intense or recent are most likely to be activated, since those latent impressions will be more powerful than the latent impressions of less common, less intense or less recent associations.

How we can program,unprogram this unconscious habits or manasa.? The effort to reprogram the unconscious is called samma vyayamo. and samma sankappo (setting right intention) is one of the method. There are 4 types of effort
(1)Maintain (2) prevent. (3)arouse (4)abandon. Maintaining and preventing can happen from the very beginning till it get form of thoughts. One need strong mindfulness to detect it early and process so. till it becomes habit.

After abandoning it is very help to set intention to prevent it next time as early as possible. similarly for arousing it is very help to set intention for the same. setting intention is samma sankappo ,vaci sankhara . which are initiated by vitaka and vicara.

Sense restrain (samvar) is also great practice in buddhism, to delete old habbit pattern or over indulgence with sense object.

When you sit for meditation, you resolve to pay attention only at the nose. This will direct the unconscious attention process to confine to nose area. however due to short attention span it will shift on its own to distraction object. as soon as you realise the distraction gently bring it back.this mean again set intention to pay attention only at nose. repeat this setting of intention all the time and train yourself. after many months of practice unconscious attention shift will get minimised.becomes a habit.

you can also indirectly train the unconscious behaviour by following.

1. analysing unwholesome mental object ,,
2. thining its drawback, danger, escape and solutions etc
3. analysing wholesome mental object thinking its benefits and methods to cultivate.
4. studying dhamma , discussing with other practitioner etc.. exploring the points where you are mis understanding the teaching. etc
5. changing mindset, building sanna, like asubha sanna, etc.
6. developing to see tilakhana .

Abhidhamma is the object of AdhipannaSikkha which are meditated as 8 knowledge, 8 Vijja (see DN10 SubhaSutta and Abhi. commentary), so it's impossible to explain Abhidhamma to new or medium practitioner without mistake. It's for people who has little defilements because they are ready to see the smallest and quickest elements' arising and vanishing without bias.

So, for me this question is like "please explain Theory of Relativity to me in simple term and short".

Whatever... just try.

Mind and mental elements are the quickest arising and vanishing, more than trillion times per second.

Physical body are 17 times slower than mind and mental elements.

17 mind and mental elements vanished is equal to 1 physical body vanished.

So, nothing is changed because mind is quickest so it has only one chance to take only one object before vanished immediately. Every mind never change it's object, one object only.

It's like this through all trillion times per second of mind arising and vanishing.

Then...

Each mind has difference origins from other minds, even the same type of minds. The difference origins are the specifier of their effected mind.

when we meditate on breath then the object of attention is breath.. and after some time suddenly a past memory image pops up.

In trillion times of minds are arising and vanishing per second...

• The originated-by-Karma-mind are knowing past-life's dead moment's object. This object appeared to mind because of Karma.

• Some karma-doing mind are knowing breath. Breath appeared to mind because of the past attention originating this karma-doing mind moment as the habitual condition (Pakatupanissaya Paccaya) which means recurrent habit. The practitioner has done breath meditation past lives.

• but some are knowing past memory image. Memory image appeared to mind because of the past attention originating this karma-doing mind moment as the habitual condition (Pakatupanissaya Paccaya) which means recurrent habit. The practitioner has not strong enough current breath meditation, so other conditions originate other effects, other mind, to arise and know memory image instead of knowing breath mind.

It's really deep more than that actually. That's why we need to meditate Jhana in advance before meditate the insight meditation.

I don't think there is one understand what I've written however I confirm that what I have written are understandable but the listener need to meditate Jhana in mastery level first.