You get adrenaline by either killing your opponents or collecting Adrenaline Pills that are throughout each map. Once your adrenaline reaches 100 it will start to flash and the announcer will tell you that your adrenaline is maxed. You now have 4 special moves that are available to you. You can activate these by doing special move combinations.
D, D, U - Agility: You will run faster and jump higher when Agility is activated. This adrenaline move works extremely well in CTF and Bombing Run, letting you escape death quickly.
U, U, U - Berserker: When Berserker is activated you will deal more damage to your opponents, similar to the Double Damage. This is a good move to use in Deathmatches and Survival.
D, D, D - Regeneration: Regeneration will regenerate your health and your shield. You shield will max out at 150, but your max health depends on your race. This adrenaline combo works well for every situation. You can never have too much health or shields. When you have the flag or the ball, this can extend your life long enough to score.
R, R, L, L - Invisibility: The only 4 button combo makes your player almost impossible to see. There's no better way to sneak into an enemy's base. You are not 100% invisible so an opponent with a sharp eye can still catch you sneaking by.
As soon as you activate your adrenaline it will start to count down from 100. Your powers are only good during this countdown. Grabbing more adrenaline while it is counting down will extend your time out a bit also.