ymelup talks about a centrifugal clutch that engages the drive when the rotation speed increases above a certain amount - such as is used in chainsaws.
Centrifugal clutches can also be built the other way round, so the clutch disengages the drive above a certain speed.
This is often used in road cars to prevent the car over-revving - when you hit the red line (or often a little above the red line) a centrifugal clutch kicks in (kicks out?) to disengage the drive and prevent you damaging the engine.