Perception of breathing
Deep, slow and rhythmic breathing is an essential condition for regulating the mental process. Very slow inhalation and complete exhalation (by the use of diaphragm) is called dirgha svasa (deep breathing or diaphragmatic breathing).
Complete awareness of breathing and nothing else but breathing is the basis of svasa preksha. Attention can be kept focussed on a single point in the respiratory tract, e.g. nostrils, or it can travel the entire tract during inhalation as well as exhalation. Various facets of breathing such as movement of the diaphragm, rate of breathing and depth of breathing can conveniently become the object of svasa preksha.
Svasa preksha can be practised in two ways, through dirgha svasa and samavrtti svasa.