Underwater pressure demonstration using water bottle - GIF

[Frank S]

Hyper-oxygenation is a common but also risky process to prolong your time under water. You have to be VERY disciplined, aware of your limits and the subtle signals your body sends you. Our normal breathing is triggered by CO2 level in the lungs. When you oxygenate, you purged the CO2 from your lungs. It is then possible to depleted too much of the O2 before you body tells you to breathe. You can easily pass out from O2.

If you pass out on land your body will soon begin breathing again. If you pass out underwater the outcome is much worse.

Military diving instructors used to purposefully have their class to take rapid too deep breaths until everyone hyperventilated and felt lightheaded dizzy and or passed out while sitting in the classroom, once everyone came around, they would go into the spiel of why that happened how to recognize the symptoms and to imagine what would have happened had they dove with the onset of the symptoms. Then stress the point of never taking more than three moderate deep quick breaths before your dive breath.
Not sure how they do things now days its been over 50 yeasrs ago for me.