A kriya (cleansing) exercise that purifies the sinuses, lungs, blood, tissues and abdomen.  On a subtle level it works on the Ajna, Manipura and Sahasrāra Chakras.
 The emphasis is on the exhalation.  How to practice? The abdomen is drawn inward toward the spine on the exhalation working together with the diaphragm which moves upward from the abdomen to force out the air in the lungs.  This forms a vacuum in the lungs which then automatically sucks in the air to equalize the internal and external air pressure.