Exams. Once I go into an exam room the adrenaline starts pumping and I go into super mode. Questions I usually struggle with suddenly become as clear as day. I end up acing everything.
Yeah, I power-up and become unstoppable when I have a short time limit. Of course this applies to assignments as well if I put them off for long enough, so I do just as well on them, but I prefer exams because they take less total time.