How do you activate the combos? I've tried but I can't seem to do it
When you have 100 adrenaline, press the appropriate keys in quick succession (L=left, F=forward etc). When you activate a combo, your adrenaline immediately starts draing back to zero, but you can make the combo last longer by getting kills/caps.
Speed (a short burst of speed and high-jumping that lasts until your adrenaline runs out- which is *very* quickly in UT2004) FFFF
Booster (regenerate health up to 199 then shields up to 150 at a rate of five per second until your adrenaline runs out) BBBB
Invisible (become almost invisible while the adrenaline lasts- you're still just about visible, but very hard to spot/aim at. Arguably the most useful combo, especially in instagib) RRLL
Berserk (increases weapon firing speed while the adrenaline lasts) FFBB