It is the free wheel which prevents a bicycle from moving backwards when we pedal in the reverse direction. In the positive drive, the rollers are pressed against a spring pad. As a result the rollers are wedged between the driving member (outer disc with the chain on it) and the driven members (inner axle connected to the rear wheel). So the force applied on the outer disc is made to rotate the wheel. The mechanism is such that when pedaled backwards, the outer disc simply slides over the axle and the drive is disconnected. This s called freewheeling and the mechanism is called freewheel or over-running clutch.