High pressure *air*, I forgot to type the last word. Basically what it is. It uses high pressure air to cycle the pistol and propel the round, instead of green gas/propane. Gives better performance in the form of consistency, since it doesn't suffer from "cool-down".
When you shoot a propane pistol, the gas has to expand to shoot the round. The expanding action requires a lot of energy, and that energy doesn't pop from nowhere, as from the laws of physics. That energy transfer causes the parts and gas to cool down a little each shot, and since the gas is now colder, the expansion diminish, therefore reducing the power output.
Using air eliminates the cool-down effect, and gives better accuracy because the power output is always the same.
The downside of HPA is that it requires an external air source. One example would be paintball markers.
Edit: Again, I'm too slow.
Originally Posted by lurkingknight
It's not a great idea for pistols since you have to tap the magazine to do it, but for rifles you run the air line through the pistol grip, it's a lot more mobile since you run it on a remote line to a tank on the belt. I suppose you could do it with a pistol as well, but it seems more clumsy to me.
Well, since this is IPSC, and they probably don't reload on stationary target shooting, it's a good tradeoff for consistency.