From a shallow water black out perspective; how important is is it to re-breath the air in your mask upon ascent?
My understanding is that upon ascent your lungs expand, effectively vacuuming the oxygen out of your system. If this is so, then it makes sense to me that upon ascent re-breathing all the expanding air you have blown into your mask during descent would act to minimize this "vacuuming" effect.
Does anyone have more than just a guess as to how wrong I am?