A twistgrip is a handle that can be twisted to operate a control. It is commonly found as a motorcycle's right handlebar grip to control the throttle, but it sometimes found elsewhere, such as on a bicycle as a gearshift, and in helicopters.
Motorcycle throttles are spring-loaded to cut the engine power back to idling when the twistgrip is released. Formerly some motorcycle throttle twistgrips had a screw that could be screwed in to make the twistgrip stay still when released (e.g. for the rider to signal right turn), but a ruling from Brussels forbad that on safety grounds.