[trux]

It does not really matter whether it is during static or a deep dive. The reflex is based on the facial thermo-receptors, and the effect is almost immediate. It does not need to be before the dive only. And since typical diving mask covers significant part of the face, removing it will definitely have positive effect. Well, if you already dive with just goggles, then it won't have any big impact, but otherwise it may indeed help. I agree though that I wouldn't recommend it trying just when you are in an imminent threat of BO - you must be used to do it relatively automatically, so that it does not cause more harm than good.

And to Jome - yes, I believe we discuss here recreational, exploratory freediving or spearfishing. It is certainly more or less pointless for competitive freediving. And yes, normally you should never need it, but you never know - if you get tangled or have any innexpected situation, having drilled such methods regularly may save your life. If you do not release the weight belt buckle automatically at most dives, you will certainly not think about it when you are in panic.