I recently asked about underwater swimming and the topic of hyperventilating came up. If I do a 5 minute session of hyperventilating twice a week prior to swimming is this likely to have any significant effect for good or bad on my fitness?
Hyperventilation is a legitimate method for increasing the proportion of oxygen in the blood for the purpose of things like holding one's breath. However, there is a caveat to it, namely that the body tracks both carbon dioxide levels and oxygen levels, and with what's essentially two different mechanisms. Hyperventilation decreases CO2 faster than it increases oxygen, which means extensive hyperventilation can lead to a situation where, while holding your breath, your oxygen levels drop too low, leading to a black-out, but the CO2 levels are not elevated enough to trigger the breathing reflex. This has a practical benefit in that, if it happens when you are swimming, you don't immediately start inhaling water. However, due to the different parts of the brain and when they shut down, people will often continue swimming while unconscious, and risk injury from running into walls and the like. Additionally, the period of time between when your body is starved for oxygen and when you start breathing again can lead to a situation of hypoxia, a lack of oxygen in the tissues. This generally does not cause brain damage, because the body begins to horde the remaining oxygen to preserve your brain, but the resultant lack of oxygenation in the extremities can lead to painful cramping.
More anecdotally, I've heard that repeated hyperventilation can change the point at which your body believes it needs more oxygen, which can eventually lead to not getting sufficient oxygenation because you've effectively trained your brain into believing that famine is the new norm. However, I can't find any actual scientific proof of that online, so that could be a wives tale.
As for whether it works as an exercise, I can't really say. The general use of it is instead to facilitate being able to hold one's breath, or to artificially boost performance by putting the body into a state of hyperoxygenation, done as a preparation for an exercise rather than an exercise itself.