Liam,

Oxygen deprivation and holding ones breath are not the same thing. Clinical trials show that even when O2SAT levels in the blood drop below 40% during a breathhold, there is no oxygen deprivation/debt in the brain or vital organs. There are various reasons for this, some of which I'll go into below. Suffice to say, the comparison with jumping off tall things is just not valid.

What freedivers aim for and what hockey players don't is the mammilian dive reflex. This is a combination of adaptions that kick in during a breath hold. They are the same for humans as they are for seals and dolphins, its the design of the rest of the system and the extent of the dive reflex that differs. The first to happen is bradycardia, or the slowing of ones heart rate. In non-depth disciplines (statics, dynamics) this is usually around 20-40%, while in depth disciplines, single figure heart rates have been observed. This starts happening in trained divers within thirty seconds.

Next, blood shift (vascular constriction) occurs. For me, this happens at 1:40, give or take ten seconds, if I'm doing zero-warmup dives (ie. get in, take breath, head down). If I do a warmup hold, vascular constriction will have already occurred or comes on nearly instantly. During dynamics with fins, this comes on ~40m with fins or ~35m without fins. For me and many others, the first contraction happens when blood shift starts. When I do warmup dives, my contractions don't come on until about three minutes in.

Which leads us nicely to contractions. This is a buildup of CO2 stimulating the diaphragm to contract. There are many theories on why this occurs and as you state, different people experience them at different times or not at all. To compare two of the top NZ athletes, Dave Mullins and Ant Williams, Dave gets contractions very early, at similar stages to me, yet holds the world record in dynamics with fins at 244m. He also just dived to a depth of 110m, making him the second deepest man in the history of constant weight freediving (beaten by 2m). Ant on the other hand can hold his breath for well over seven minutes with no contractions or very few. So using the logic that you can go as long as you don't have contractions (yes, I know that's not quite what you said, but I'm using creative licence a bit), Dave would be able to do 40-60m dynamics, yet Ant could do his PB of 223m. Of the various theories, the most likely is that the contractions help squeeze the blood out of the spleen (another part of the dive reflex). The end result is that unless you know your body and know how you feel on your dives, contractions don't mean jack. That's why its so important to have safeties watching any apnea trainings. Coming from somebody that's been pulled off the bottom during a training dive.

That's not half of what's known about the dive reflex, but its already a bit heavy for most, so I'll leave it there. The thing to take away though is that you're not actually hypoxic in your vital organs until blood stops pumping, not from when you start holding your breath. Repeated blackouts may cause problems, but frankly so does getting drunk. If brain damage happened during normal apnea, guys like Herbert Nitsch (airline pilot) who can hold his breath for over nine minutes would fail his cognition test that he has to take annually for his job.