Shallow Water Blackout is most commonly associated with skin diving, but the more swimming coaches I speak with, the more I have come to realize that underwater fainting is not at all uncommon in the world of competitive aquatic sports. Individuals breathing under normal conditions use the partial pressure of oxygen in the lungs. Carbon dioxide, on the other hand, serves as a stimulus to breathing by informing the brain that a person's body requires another breath of air. This process occurs in any prolonged holding of breath, including underwater swimming. Normally there is no danger because the carbon dioxide build up in the blood signals the brain that you need to take a breath by giving you the sensation that your lungs are going to burst if you don't get a breath.
Hyperventilation is the repeated inhalation of fast, full breaths of air and rapid exhalation.
The effect of hyperventilation is to wash C02 out of the lungs, resulting in an extremely low C02 level. Without the C02 stimulus, the brain doesn't recognize the need to breathe and the swimmer blacks out from hypoxia, a lack of oxygen to the brain. The blackout victim is in an extremely dangerous position at the point of unconsciousness. Seeing him underwater will fool observers as the unconscious swimmer often makes seemingly coordinated movements even after the faint comes on. He does not appear to be in difficulty.