What is my friend doing?

Factual Questions
1

While having a discussion about the strange things people can do with their bodies, I mentioned that I knew a guy who did something I couldn’t explain.

He would purse his lips real tight while biting against his thumb and blow as hard as he could. His face would turn red and the tendons in his neck would stick out from his efforts. He was attempting to blow all the air from his lungs, but couldn’t because he was forcibly holding his mouth shut. Then, just before I thought he was about to pass out, he gently exhaled and I saw smoke/vapor. I thought it was some kind of trick since he had no explanation.

Until one of the guys in the group said he could do it too! Now all my friends are amazed, but still without an explanation. Neither smoked so it wasn’t that. The only explanation seems to be water vapor or gases remaining after the oxygen has been depleted.

What is it? How does it work?

2

I don’t think this is a case of what remains after the oxygen has been depleted but, rather, water vapor that has condensed under pressure.

We had a substitute teacher who showed us how to do this when I was a kid. Very cool. He was our favorite sub, bar none.

Hats off to you, Arthur C. Farrington. You were cool.

3

I used this trick when I was younger. I would tell everyone it was cold enough to see my breath. I would then do it (not to the effort of Galyean’s friend). Everyone would then see my breath. Then they would try it. They couldn’t, and I could, and they were amazed with my superpowers. It would work in temperatures up to about 70 degrees. I am sure it would work at higher temps if I strained more, but then the trick would be given away.

4

Cool! I can do it too. Whoa…I think I’m going to pass out…