[curun1r]

It’s probably more accurate to say there’s a build back of CO2. Wim Hof uses hyperventilation to lower blood CO2 level well below normal so that when you do the breath hold, a big chunk of the time is spent just returning CO2 to that normal level.

This is in contrast with other breathing exercises to do cause an actual build up of CO2 to levels well above what feel normal. Freedivers call these CO2 tables and they’re used to build CO2 resistance and increase breath-hold times.