A method of forcing air into the lungs above the total lung capacity, employed by freedivers. Also referred to as "packing".
After completing a full inhale, the subject closes the epiglottis and fills his mouth with air. The mouth is then closed and positive pressure is created by squeezing the cheeks and completing a sort of swallowing motion, simultaneously opening the epiglottis.
Repeating this up to 40 times, the subject can force up to 4 litres of extra air into the lungs. Extreme packing can easily puncture the lungs (causing
Lung over-expansion injury), so special care must be taken in training the manoeuvre. It also causes a short (15-20 sec) but strong drop in the subject's blood pressure and can cause him to faint. It should therefore never be practiced in water without supervision.
Extra air in the lungs has obvious advantages for a breath-hold diver. Besides providing more oxygen for the dive, the overpressure of the lungs counters the pressure of the water on deep dives, thus increasing the theoretical limit which the diver can reach before the lungs compress beyond the functional residual capacity.