When you go above and beyond the usual limits.
One of my favourite examples is in Final Fantasy X where each character's most powerful abilities can only be done in Overdrive.
1. That N2O system put that car into overdrive!
2. Whoa! He's gone into overdrive! Run!
Overdrive is when you get hyper-active due to being over-tired, over-worked, or over-stressed. Common symptoms include slurred or incoherrent speach, laughing histerically over stupid things, and improper brain function, while in a sober state.
J: hey man how's it going?
K: whats the dealy-yo my mizzle muahhahaha, like, ur face is stupid hahahha!!!!
J: dude, you're in overdrive.
You know you're in overdrive when you haven't had any sleep in a long time and suddenly your fatigue is taken over by this crazy ass energy that makes you act stupid.
Jeremy: "Hey, what's up KJ Can you pass me some more of that KD?"
Karl (who hasn't slept in days):"uhhh hahahaha what the fuck? haha i'm going crraaazyyyyy"
Jeremy:"DAmn man.. you must be on overdrive"