"Pranayama is not just automatic habitual breathing to keep body and soul together. Through the abundant intake of oxygen by its disciplined techniques, subtle chemical changes take place in the body. The practice of pranayama regulates that flow of prana throughout the body. It also regulates the body's thoughts, desires and actions, give poise and the tremendous will-power needed to become a master of oneself." Light on Pranayama, B.K.S Iyengar
This technique is used to balance the male (right side or sun)/ female (left side or moon) energies of the body and cultivates peace and stillness. This also cleanses the nasal passages, balances high blood pressure, improves awareness and concentration, and when you expand the lungs and chest, you are opening the heart to gratitude.
Sit in a comfortable easy seated pose, Sukasana or other mediation posture.
Inhale deeply through nose, filling the lungs full; Exhale deeper.
Allowing the tongue to role back, inhale through nose, followed by an exhale with both nostrils partially closed for 3 rounds.
Or in Vishnu Mudra, alternate one nostril, then the other, while lengthening the breath.
To do this: Inhale through one nostril, apply pressure to close the other nostril.
Pause the breath.
Then, change the closure to the other side, exhale through the other nostril.
Practice slowly for 5 to 10 rounds with concentration.
Emphasis is in the exhale, which can be longer than the inhale. (Although, Anand preferred the inhales and exhales the same length in a 1:1 ratio. "Progressing only when there is no more discomfort". i.e. 1:2, 1:3... )
Vishnu Mudra: Placing the thumb on one nostril, the pinkie finger on the other and the middle fingers to the 3rd eye.
Caution: Do not practice this technique with heart conditions and only after Ujjayi and Viloma breaths are mastered.