There's an interesting caveat here, the phenomenon is well known to apnea practitioners:

- Hyperventilating, then holding your breath underwater can easily be deadly.

- Doing the same without hyperventilation is much safer.

The difference lies in the fact that our ability to sense our limits are almost entirely due to CO2 buildup. We actually almost can't tell when we run out of oxygen, it's why pilots are fine one second and pass out the next when they don't notice that the oxygen runs out (https://www.youtube.com/watch?v=kUfF2MTnqAw).

Hyperventilating reduces your CO2 but leaves oxygen saturation almost unaffected, which for an unexperienced person is equivalent to doing a blind walk next to a cliff, and removing the barrier.

The reminder is never ever practice these techniques alone or at the same time as someone else. You need to have someone nearby who is aware of the practice, and can monitor your status at all times.

https://www.chicagotribune.com/nation-world/ct-navy-seals-dr...