The video is showing something called a freediving blackout, it’s a condition that gets triggered in divers such as the one shown, plus other factors such as hyperventilating prior to the dive.

What happens in this case is the person loses consciousness because they don’t feel the need to breathe as they have purged a lot of the CO2 the body has prior, giving them “room” to not feel the urge to breathe.

Again, as specified previously, assuming the person doesn’t die of anything immediate in the water, the body simply stops breathing.

The problem is when it starts again.

Your body, once “jolted awake” by the involuntary nervous system, will try to breathe, and it doesn’t know or care that you might still be underwater since you stopped swimming and didn’t get out of the water in time or weren’t rescued or washed ashore on some deserted island.

If you’re out of the water at this point, you’re fine, usually. If you’re still underwater, you breathe in (involuntarily) a bunch of the water (or a bunch of “nothing” if you got your holes plugged) and that’s what kills you.

Kind of ironic that in an attempt to preserve your life, your own involuntary bodily function is what ends up being the final nail.