Originally Posted by monkeyracing

That's a spool. Loads of those around. I've got DIY, Yeah Racing and Tech Racing versions.

The answer, I think, is Tamiya. They made a one way for one of their awd cars that fits the stock mini gear diff housing. I've got one and it's always an adventure using it. Locks up on throttle, unlocks off throttle, zero braking! WHOO!