Using a 'kiai' of some kind is extremely empowering. You can kick, hit and throw incredibly hard if you just YELL at the same time that you do it.

Also, it can psychologically disempower your opponent.

Try it someday! Next time you're chopping wood or breaking something in a fit of frustration, go "eeeeeeyAAAAAAHHH!" when you do it. Unleash some raw power. It works wonders.

There's no particular word that's "supposed" to be used, but "hyah" just comes naturally, I guess. You've got the build-up from the quiet "hy" part, then the release from the bellowed "-AH!"

Edit:
[q=Sepho]I'm thinking it's the same idea behind powerlifters screaming when they lift.[/q]Exactly.