One trick they shared was they always left room so when they felt done they could suck in a tiny bit more air. Obviously that doesnt help an under water thing but similar to storing it in cheeks.
Holy shit I just did it sitting on my desk. WOW. Storing in the cheek = free tank for another breath. It's like doing holding breath TWICE. Holy hell. You just need to do this carefully so it makes sense:
1. I noticed your cheek cannot expand because they're not muscles. They can only expand when you close your lips so your cheeks can expand against the seal.
2. That means when you're inhaling deep before holding breath / going underwater,
do it through your nostrils while your mouth is closed tightly.
3. As your lungs get topped off, THEN open your mouth and also simultaneously inhale then close lips + expand cheeks.
4.
That is your oxygen tank.
5. Now you're holding breath / underwater.
6. Exhale whenever you want since it doesn't have any benefit to hold CO2
7. When you're out of breath but still not breathing, you can now just wait as long as you can. Come out when done.
BUT WAIT, you can now ignore last sentence of #7 and go to the bonus #8
8. As you feel terrible burn from lack of oxygen, you have to carefully try to inhale underwater ever slightly without actually have the water go up your nose all the way (flirting with the water floor line inside your nostril by 1-2mm or so).
9.
As you do that, contract your previously expanded cheeks slowly.
10. Then you feel a moderate relief of breath holding because your lungs just consumed some oxygen from your cheeks.