[hcarvalhoalves]

This exercise shouldn't be done underwater. This is an exercise for being aware of the diaphragm reflex, to go thru the sensations without panicking, which helps you to stay relaxed and develop a proper routine while diving.

By relaxing and exhaling you post-pone CO2 saturation as much as possible, but it still happens before O2 deprivation (it's only a problem when you reach <85% O2, and it takes a while for that).

Here's some great footage of a native from Philippines spearfishing the old way, showing a lot of self-control:

https://www.youtube.com/watch?v=YDspP4BhlTw