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.