This exercise introduces kumbhaka into your breathing practice. Kumbhaka is retention or suspension of the breath. It is the phase between inhalation and exhalation. Kumbhaka is the confining of the prana inside.
"Respiration being disturbed, the mind becomes disturbed. By restraining respiration, the yogi gets steadiness of mind." Hatha Yoga Pradipika.
This exercise is a combination of viloma, interrupting the exhalation and kumbhaka, retention of the exhalation. Maintain receptivity throughout the body, release tension and strain, and be careful not to agitate your mind by attempting to over extend the length of your retentions.