http://en.wikipedia.org/wiki/Shallow_water_blackout
If anyone remembers a few years ago someone drowned in Australia due to hyperventilating before they swam underwater in a backyard pool. Here’s the reason.
In summery your body is already saturated with Oxygen (O2) at normal resting levels and returns to these levels quickly after you’ve begun breathing after holding your breath. Hyperventilating thus only reduces the amount of Cardon Dioxide (CO2) in your body and doesn’t add to the level of Oxygen your body has in reserve.
Blacking out is determined by the level of Oxygen in your blood and your body uses Carbon Dioxide to determine when you should breath. This hyperventilating before holding your breath means that you’ll pass out before you feel the urge to breath :(