This exercise is called Uddiyana Bandha and has been used as a abdominal tonification and purging exercise in yoga for hundreds of years. Recently in the world of freediving the effects of diaphragmatic manipulation in this way has been proven to improve the elimination of air from the lungs and thus enables the ribcage to flex and compress more with the more extreme pressures at depth when the lungs have been reduced to Residual volume. Simultaneously with the reduced volume and the increased flexibility the trained freediver is able to “pull up” a little more air from the airways for deep equalisation.
This exercise must be practised carefully on an empty stomach, and regularly to reap the benefits. However a word of caution, just with this practice alone it DOES NOT mean that a freediver who has a good equalisation technique should jump irreverently to depths without a well prepared training program. The body needs time for physiological adaptation and the rush for depth must not be at the expense of safety.
Injuries that can result in irreversible damage are pulmonary barotrauma, i.e lung squeeze and trachea squeeze, that can leave scarring and result in limiting your depth freedives permanently. ALL UNNECESSARY AND COMPLETELY AVOIDABLE.
After all, Freediving is about fulfilment, exhileration, human potential and absolutely about humility!