[BlameKaneda]

Breathing into a paper bag is a stereotypical "calm yourself down when panicking" technique, but I've done it mid-panic and it has had an effect on me. With that being said, it's debatable as to how well it works.

[aaaaaaaaata]

Depriving yourself of oxygen and increasing CO² does have a sedative effect, yes.

Might also be the focus on one's breathing that helps one detatch from the stressor.