Press the perineum with the left heel while stretching the right leg forward on the ground and holding the right leg firmly with both hands.
Perform jalandhara bandha at the throat, retaining the air upwards, even as a snake beaten with a stick becomes straight like a stick.
The kundalini quickly straightens and then the breath ceases its function in both the nostrils.
Exhalation should be slow and gradual. The wise call it maha mudra. This maha mudra is able to destroy all kinds of pain and even death.
One should practise the mudra on the left side and on the right side. When both become equal, then the practice should be left.
By its influence even faulty diets are easily digested and poison can be digested like nectar. Ailments like tuberculosis, leprosy, piles, indigestion, and other diseases are destroyed by those who practise maha mudra.
This maha mudra can give great siddhis to men, and should not be given to ordinary persons.
Hatha Yoga Pradipika (3:10-18)