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As it was explained to me, the purpose is rather mundane. During some kinds of meditation, esp. when one penetrates lower chakras, there is lots of saliva generated, swallowing which can be quite a distraction and interfere with abdominal breathing. Raising the tongue makes most of saliva flow back into the throat instead of filling the mouth.
P.S. in a commentary to ancient Chan text "Lung Chung" (The Brief Precepts) it says that pressing the tongue to the roof of the mouth eliminates the objects of discursive (speech-type) thought.
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Many meditation techniques work by over riding the autonomic nervous system. I find this technique effective at 'thought stopping'. Like many meditation practices you must practice the technique to benefit.
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What is the purpose of touching the tongue to the roof of the mouth when meditating
‘Suppose, with my teeth clenched and my tongue pressed against my palate, I beat down, hold back, and crush the mind with mind.’
So, with my teeth clenched and my tongue pressed against my palate, I beat down, held back, crushed the mind with mind. While I did so, sweat ran from my armpits.
It was just as if a strong man, holding a weaker man by the head or shoulders, were to restrain, subdue, and attack him;
even so, with my teeth clenched and my tongue pressed against my palate, I beat down, held back, and crushed the mind with mind, and sweat ran down my armpits.
But although I exerted tireless energy, and unremitting mindfulness was established in me, my body was overstrained and lacking calm, because I was exhausted by the painful striving.
But, Aggivessana, such painful feeling that arose in me did not invade my mind and remain.
If, bhikshus, while the monk is attending to the stilling of the thought-formation, there still arises in him bad unwholesome thoughts connected with desire, with hate, or with delusion, then with clenched teeth and the tongue pressing on the palate,
he should subdue, restrain, attack the (bad) mind with the (good) mind.
With clenched teeth and the tongue pressing on the palate, those bad unwholesome thoughts are eliminated and disappear.
By their elimination, the mind thus stands firm internally, settles down, becomes unified and concentrated.
Teeth clenched and my tongue pressed against the palate used to gain unremitting mindfulness in the 1st case during the Bodhisattva's self mortification. He finds himself only being exhausted by this, but in the latter case it is recommended as the last resort action drive out unwholesome thoughts.
Upvote:3
The technique is not peculiar to Tibetian Buddhism, it's also used in Zen practice -- How To Practice Zen Meditation? says,
Head And Neck
Whatever the position you choose to adopt, make sure that your back and neck stay as straight as possible. Pull your chin in a little to erect the neck and try to “push the sky” with the top of your head. Do not be too tensed or too relaxed while you do this; try to find balance in your posture. Keep your mouth closed during zazen; your teeth should be together, and your tongue should be against the roof of your mouth just behind your teeth.
(The above is just an extract, see the link for other detail).