The word “khechari” is derived from 2 Sanskrit roots that are translated as meaning “one who moves the sky.” An ardent practice of this mudra propels the devotee into a state of the universal awareness of God.
Kechari Mudra - method
1 Sit tall, close the mouth and roll the tongue up to touch the upper palate. Initially it may touch the hard palate. Some may be able to touch the soft palate immediately; retain as long as comfortable.
2 When the tongue starts to feel uncomfortable, release it back to the normal position.
3 Rest and repeat. This can be done when you are engaged in light activities while sitting and walking, as well as meditation.
4 With practice you will be able to hold the tongue on the palate for a prolonged duration, and the tongue will be able to go further back.
Benefits
1 Gheranda Samhita says that by performing Kechari Mudra, the yogi overcomes fainting, laziness, thirst and hunger.
2 Strengthens the immune system, delaying disease and decay; said to have anti-aging properties.
3 Kechari mudra helps the practitioner to attain Samadhi, the super-conscious state; the body becomes divine.
4 Considered the king among all the mudras and is given great importance in all the ancient yogic texts. Helps to detach the astral body from the physical body and travel in the astral planes.
https://www.youtube.com/watch?v=KBJbmUH4iG4 This is a really clear, simple instructional video