The Hatha Yoga Pradipika states in 3.32-36, that Khechari mudra is turning the tongue backwards into the cavity of the cranium and turning the eyes towards the eyebrow center. Yogis go to the extent of making the tongue flexible (they cut the frenum gradually over a period of time) to gain the best effect of the mudra. We don’t have to go through that process. All you need to do is turn the tongue inwards and press the under surface till it touches the upper back portion of the soft palate. While other descriptions state that the tip of the tongue needs to press the soft palate, the HYP states the tip should reaches the nasal orifice at the back of the throat. You will need to release the tongue if it tires, but with practice you can hold longer. This mudra is practiced with other techniques such as Ujjayi or meditation.
Verse 3.38-39 states (literally) that the yogi who remains with the tongue upwards for even half a second is freed from toxins, disease, old age, hunger, thirst or unconsciousness.
In this mudra the pineal gland located at the center of the brain and ajna chakra are stimulated. The psychic center, Bindu visarga, at the top of back of the head, is also influenced by the mudra. Bindu, when it is full, releases its nectar down to the entire body. It is easy to see from the picture, the proximity of the center of the brain to the inside of the ajna chakra (space between the eyebrows).
Khechari mudra controls/influences the network of endocrine glands throughout the body. This is done by regulating the powerful secretions in the brain. The glands influenced are thyroid, mammary, thymus, adrenal and reproductive glands. The practice of the mudra influences the centers in hypothalamus and brain stem which control autonomic breathing, heart rate, emotional expression, digestion, etc.