Ujjayi breathing is used primarily in Ashtanga Vinyasa yoga for a number of reasons. The sound that is created provides a focus for the mind which helps with single-pointed concentration and drawing the attention back to the breath during the practice. It also creates heat because it takes energy and concentration to preform the breath and because the back of the throat,the glottis, is gently contracted during both inhalation and exhalation. This narrowing means that the same amount of air that was traveling through a larger opening is now passing through a smaller opening which causes friction or a building of energy.
The heat that is produced is an internal heat that helps to open the body and lengthen the muscles from the inside out. Uddyana Bandha is a gentle drawing in and up of the navel to the spine underneath the rib cage. This activation tones the deep abdominal muscles that wrap all the way around the torso and attach onto the spine at the back of the body. This engagement helps to provide and build support for the whole core body. When preformed correctly it feels like a deep containment of the torso all the way around, both front and back. On the inhalation this muscular activation will need to be relaxed slightly in order to take in a full breath.