I was just wondering about something..

In the trickjumping movie Tribulation(insane), they sometimes bounce.. when they fall down from something, and they land on the ground, they just bounce!? Dunno how to explain it, but if you've seen ze movie, i think you know what im blabbering about.. Then, I wanna know, if it is a bug in quake3, or are they using a speciel mod? :OO