In Yoga, Kalpalakuhara is the name of a technique in which the tongue is turned back inside the mouth, raising its tip upward and fixing it into the hollow above the soft palate from behind. It is one of the mudras of hatha yoga, spoken of very highly in the ancient texts and is popularly known as Khechari mudra.
Kalpalakuhara is described in detail in Hatha Yoga Pradipika (III. 32-54) in the following words – the tongue is turned back and inserted into the kapalakuhara (skull cavity) and the gaze is fixed between the eyebrows. This mudra is also khechari. For doing this, the tongue is required to be made free by cutting the frenulum, which is a tender membrane that keeps the tongue to the floor below. That is called chedana. In Hatha Yoga Pradipika (III. 34 -36), the procedure for cutting the frenulum is described. It should be cut slightly with a sharp instrument, and a further cut should be made at the same place after the wound is healed. It takes about six months to cut the frenulum completely. Two other things are done in addition to chedana. They are calana, moving the tongue in all directions and dohana, or pressing, squeezing and pulling the tongue.
By doing these three, the length of the tongue is increased considerably, enabling its tip to touch the mid-point between the eyebrows (III.33). Then one can do the khechari mudra by inserting the tip of the tongue inside the hollow space where the three nadis, ida, pingala and sushumna, meet. That place is called tripatha or vyomacakra (III.37). It is said that by practice of Khechari mudra, one overcomes the ill-effects of poison, as also disease and old age (III.38). The bindu (semen) may be preserved even when one is embraced by a woman (III.42).
Describing the importance of khechari mudra Svatmarama says – the adept who, by doing the khecari mudra, drinks the ambrosia secreted by the moon situated at the top of the skull (talu) can overcome death in two weeks. His body always remains fresh and perfect, and even if he is bitten by the deadliest venomous snake like the king cobra, the poison does not affect him. Just as fire, which has plenty of fuel or a lamp with plenty of oil, keeps on burning indefinitely without being extinguished, the adept doing khechari keeps on living and does not die as he is invigorated by drinking ambrosia from the moon (III. 44-46).
Svatmarama likens the tongue to cow flesh and the ambrosia from moon to alcohol and says – he is the most pious of men who always eats in the form of the tongue placed inside the skull (talu) and drinks in the form of ambrosia from the moon. He compares the two mudras, khechari and sambhavi, and says that the effect of both is the same, that is, liberation through silencing of the mind and breath.
More or less the same description of khechari mudra is found in many Yoga Upanishads such as Dhyana Bindu Upanishad (79-85), Yogakundali Upanishad (II.28-49), Yogadamani Upanishad (52-58), Yogatattva Upanishad (177) and Sandilya Upanishad (I.14-19).