What is hyperventilating and extended breath holding?

Hyperventilation is a series of deep breaths followed by forced exhalation prior to breath-holding. This is done in an attempt to remain underwater for a longer period of time. This works because it decreases the level of CO₂ in the blood. CO₂ is responsible for triggering the need to breathe. With less CO₂ a swimmer will not feel a need to take a breath as quickly and can remain under water longer.

This however, does not mean that the swimmer does not need oxygen. Oxygen levels are being depleted. In fact, oxygen levels can be depleted more quickly if the swimmer is moving or swimming rather than remaining stationary. If oxygen levels in the blood drop sufficiently before CO₂ levels trigger the need to breathe, the swimmer will become unconscious. This is also commonly referred to as shallow water blackout. Obviously, an unconscious swimmer will breathe water into their lungs and will die if not rescued and resuscitated.

What National Governing Agencies say about hyperventilation and breath holding:

The American Red Cross
Refers to hyperventilation in its Swimming and Water Safety test as “potentially dangerous” and “risky”. The ARC Lifeguarding Manual also refers to how hyperventilation can result in a passive drowning victim and that patrons should be directed not to engage in prolonged breath-holding (greater than 20 seconds).

The Medical advisory committee of the YMCA of the USA 
“YMCA’s should prohibit extended underwater breath-holding.”

The Department of Morale, Welfare and Recreation (MWR) of the U.S. Navy
“This practice of hyperventilating and extended breath holding is prohibited at MWR aquatic facilities”.