Kapāla (skull) here may refer to: 1) the nasal cavity/sinuses, 2) the head area in general, or 3) a specific cranial cleansing technique. Vatsara with air (vāyu) seems to imply air movements that cleanse the cranial region.

The description — filling the abdomen with air and expelling — suggests that the air moves upward, possibly exiting through the nostrils in a vigorous exhalation, cleansing the paranasal sinuses.

This practice is related to kapālabhāti, though in Gheraṇḍa’s system they are distinct techniques.