when you press the gas the rpm of the motor speed up thus spinning the shaft faster. centrifical force pulls the shoes in the clutch toward the drum wall, and engaging the clutch. the drum wall is attach to the chain and that is attached to the rear sprocket. the sprocket is bolted to the wheel, so when the sprocket is spun by the rising level of rpm it push's your gokart or mini bike forward. the reason why you sproket keeps turning when your not pressing the gas, is because its bolted to the wheel. = when your moving, your wheel is moving, which means your sprocket and chain and clutch is moving, even when their not engaged. i hope this clears this up.