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.