Hatha Yoga Pradipika has defined this pranayama as
mukham samyamya nadibhyamakrsya pavanam sanaih,
yatha lagati kanthattu hrdayavadhu sasvanam,
puravatkumbhayetpranam recayedidaya tatha.
Controlling the mouth and partially closing the larynx, the air should so inhaled through the nostrils that a fine rubbing or hissing sound is produced from the throat down to the lungs; holding the breath, the air should be exhaled through the left nostril. Inhale with the both nostrils slowly producing a hissing sound. Retain the breath. Slowly start exhaling through the left nostril by closing the right nostril. Prolong the exhalation as long as possible with ease while enjoying the hissing sound. Repeat the 9 rounds.
This pranayama is useful to calm down your mind. Helps in relieving insomnia and therefore should be practised before sleep.
Also it is helpful for high blood pressure patients.
Disclaimer: The above pranayama should be done under guidance of qualified yoga instructor.