Nitro-powered RC cars come with clutches, usually with either 3 or 4 shoes, that automatically engage when the engine gets to a certain rpm range.
The clutch bell is an important part of this. On the diagram on the right you'll see the flywheel of an engine. You attach your clutch shoes and springs to the flywheel.
Then, the clutch bell (with its attached gear) goes over the shoes and springs.
When the engine starts revving, the flywheel and clutch shoes start rotating, but the clutch bell is initially still (the car is still.)
As the rpms pick up, the shoes pivot outward and rub against the clutch bell, forcing it to rotate, and move the car.
When you slow down again, and the engine rpms drop, the clutch shoes will again disengage, allowing the engine to keep rotating while the car comes to a stop.