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"
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!