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.
You are probably well aware, but always worth highlighting the risk of shallow water blackout and death if you do this wrong and unsupervised. Always have a dive buddy.
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.
Can you elaborate? As others are saying this can be extremely dangerous. Normally I wouldn't just reiterate other comments but if it might save a life can't hurt to be sure.
That is still hyperventilating in a technical sense, as is the breathing you're doing for a minute beforehand; both will result in lowering your CO2. If you're actually holding your breath and going under water, please take an intro freediving class. So many people die every year just because they don't have the basic safety training necessary to turn this activity from extraordinarily dangerous to quite safe.
Lets me hold my breath for 4–5 minutes if I don't move too much.