NEVER HOLD YOUR BREATH WHILE UNDER WATER!!!!!
Ohh wait.. freediving sorry.
I heard a lot of people will practice during the day at work just by holding their breath.
Another thing, the mental aspect of it. Your brain tells you that it needs air and you automatically want to breath, if you train your mind to hold off on that feeling, you can actually hold your breath longer. It just takes time and practice. Your brain has just been trained to tell the body to breath in and out, in and out... it can be trained again...
Makes sense? I'm not a freediver but, I read a lot about it.
Michael