First you press (make sure you learn the order) 8, 9, 4, 5, 1, 9 (with commas included) - then turn in a circle three times (clockwise) and jump twice. Finally bring the console down (with `) and type /quit - normally this would exit the game, but for some reason there is a glitch/easter egg where if you have done the above correctly, and fast enough it enables the bug rather than closing the game.
The devs have no idea how this work - apparently its an old old bug/easter egg hidden in the quake3 engine that changes skins - thats why they havnt fixed it yet. Give it a go, its a good laugh!