Sit in padmasana. Inhale slowly and deeply. Retain the breath. Close the ears with the thumbs, eyes with the index fingers, nostrils with the middle fingers and place the ring and small fingers above and below the lips to close the mouth. Concentrate on bindu chakra while retaining the breath inside. Try to perceive any manifestation, and breathe as long as possible. Continue this same process a few times.
This is a powerful practice for withdrawing the mind from association with sense objects (pratyahara). It stimulates awareness of psychic sound, which emanate from bindu chakra in the back of the head. This is a technique of nada yoga.