The carbon dioxide tolerance test involves inhaling deeply through the nose, exhaling through the mouth four times, then taking a fifth deep inhale and exhaling as slowly as possible through the mouth.

“what you're going to do is you're going to inhale through your nose as deeply as you can, you can do this lying down, sitting, whatever inhale through your nose and then exhale all the way. So that's one. You're going to repeat that four times. So inhale, exhale, inhale, exhale inhale, exhale, inhale, exhale four times. And ideally you're inhaling through the nose and you're exhaling through the mouth. That's just the beginning of this carbon dioxide tolerance test. Then you take a fifth inhale as deep as you can through your nose. Fill your lungs as much as you can, and if you can try and expand make your stomach go out while you do that, that means that your diaphragm has really engaged. So you're inhaling as much as you possibly can. Then hit the timer and your goal is to release that air as slowly as possible through your mouth.”