Quake 3 Arena Question

[Quakepc]

I know quite a bit about Quake 1-3 in general. However I notice time to things I am unaware of. I noticed some players are able to hover/fly around on really almost any Map. Really ruining my day! I see 'death from above' frag my ass. It's very hard hitting a fly with any accuracy. Only to get fragged while trying to get a shot into heaven. lol

(+) is this a legit thing or?

[Quakepc]

Figured that out by accident when I jumped in a new skin during Quad Damage. I always used sarge and never paid mind to others. I always thought it was a Mod and I didnt assign a key to the 'feature'. I always thought a skin was a skin as in Quake & Quake 2. What else do I not know on this topic?