I have used 3-7 breathing to prepare for diving. Inhale 3 seconds, exhale 7 seconds, for at least a full minute, followed by hyperventilating 5 times.
Lets me hold my breath for 4–5 minutes if I don't move too much.
Lets me hold my breath for 4–5 minutes if I don't move too much.
It effectively removes the normally leftover carbon dioxide from blood, but it does not oxygenate blood significantly more than normal.
The end result: if you hold your breath after hyperventilation and start doing physical activities, you can get dangerously deoxygenated blood. Without the usual feeling of asphyxiation that is normally triggered by high CO2 content.
Deoxygenated blood + brain = fainting. Which can be lethal when swimming.
Someone I went to school with almost died from this. Was in a coma for 48h and spent a month in hospital afterwards. And that was in a public swimming pool where he was discovered quite quickly.