> > > And shouldn't the potentially diminished O2 content in the bag actually increase, rather than alleviate, hiccoughs?
>
> No idea. I'm just repeating what I was so recently taught :-)

I've heard that the reason this sometimes works is nothing to do with the O2 content, but solely because the bag helps with the "control your breathing" hiccup cure. If you tell somebody to do slow, regular deep breathing and they're hiccuping frantically all over the place, chances are they won't. Tell them to breathe into a bag, however, and it gives them something diversionary to focus on. The regular deep breathing then tends to happen automatically.

Whether or not controlled breathing really HAS an effect on hiccups is another question, to which I don't know the answer.