What should I do when my cat does this?

You know how sometimes cats get into a respiratory spasming thing like
they're trying to clear something from their throat (a hairball or
something??) I don't know what to call it. It's very rhythmic, and it
appears to be involuntary, like they're choking on something and they're
trying to expel it. Every cat I've ever had goes through this from time to
time. They're obviously in distress. I want to help and I'm thinking
maybe I should slap them on the back to help "dislodge" anything that's
stuck there, but I don't know if that would help or hurt. I'd love to know
what's going on and what I can do about it, but it never occurred to me
until now to go online to a cat interest group and ask!
So I'm asking. Does anyone know what that spasming is called, what causes
it, and what we humans can do to help our little furry friends get over
their episode?
-Garret Swayne
garret at garretswayne dot com