Also known as a steering wheel spinner, a "wrist breaker" is a device made to attach to a vehicle's steering wheel to give the operator one point of contact with the wheel so it can be rapidly turned. The wrist breaker earned it's slang name through urban myths about people using them in their cars having broke their write when their car's steering violently jerked while going over extreme bumps.
"damn son, that's a dope wrist breaker on your whip."
A wristbreaker is a large test or hand-written essay that requires an unusually large amount of writing compared to the average. You know you have a wristbreaker when the next day, it looks like you injected your wrist with steroids.
Joe: Bob! Holy crap, what happened to your wrist?
Bob: I had a wristbreaker yesterday in history class, and no my wrist muscle is jacked.