Interesting. Got a good source? I should definitely look into this again.

https://www.divessi.com/en/get-certified/freediving https://www.padi.com/education/freediving https://www.freedivinginstructors.com/ https://www.aidainternational.org/Education/AIDAFreedivingCo...

I've only taken the SSI training but I guarantee none of the different freediving organizations will recommend the type of breathing you are discussing, and in fact a large part of the intro course hammers home that you don't breathe that way.

Any freediving course. You have to breathe normally for an extended time, then dive, then take an extended break, hyperventilation just makes it easier for you to get yourself killed.

The people around the pool and lake where I grew up? From what I'm reading while searching it seems that "hyperventilate" is probably a bad term for the type of breathing I was shown. We wouldn't do it until we felt a conscious change like dizziness but more like 4 to 8 good deep full and fast breaths before holding, enough to notice, but not so much that it presented the dangers I'm now reading about.