Select a specific devestator by holding your WALK key (default ALT) and pressing RELOAD (default R).
Channel your energy with meditation or channel hatred to ovecharge your force meter. It will be a purple bar that fills OVER your force bar in game as it charges. You will not be able to charge without already having a devastator unlocked.
You may perform a devestator by holding your WALK key (default ALT) and pressing SECONDARY FIRE (default RIGHT CLICK). You need to be 100% Overcharged for it to work.