Why does a swimmer hyperventilate in order to hold their breath for a longer period of time?

Answer 1

So they can hold their breath longer.

By hyperventilating they remove excess CO2 in their body, then when they take a deep breath, they are able to have more oxygen in their body, thus being able to hold their breath longer.

It can be dangerous because breathing is regulated by the level of CO2 in our body, and when too much is removed, the body doesn't know when breathe again.

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Answer 2

Hyperventilation (to a certain degree) results in a lower level of CO2 in a free divers lungs and bloodstream. This postpones the stimulation to the breathing center of the brain, thus delaying the warning signals of running out of air. It is important to note that oxygen levels of the blood are not increased by hyperventilation and is very dangerous and may contribute to shallow water blackout and deep water blackout.

Answer 3

Hyperventilating oxygenates the blood slightly more than breathing normally. This means that when you subsequently hold your breath, the extra oxygen in your bloodstream provides a few extra seconds worth of "breath holding" before the reflex that tells you to breathe kicks in.