[pcl]

you'll be free-diving like a champion

Note that this can be dangerous while free-diving. You are essentially temporarily disabling your body's CO₂ buildup alarm thresholds, which is a proxy for too little O₂. So you need to take care to make sure that you don't run out of O₂ without noticing while under 30 feet of water, possibly while weights.

The Wikipedia page on shallow-water blackouts¹ has a nice little diagram illustrating the issue. I don't know enough about freediving and O₂ consumption to really have any clue about the margins of safety typically are, but it certainly seems like something worth understanding before messing around with too much.

¹ http://en.wikipedia.org/wiki/Shallow_water_blackout

[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