The sewing machine is made to rotate by the motor turning a belt, which turns the hand wheel. Sometimes the hand wheel has a complex clutch unit built in.
Hand Wheel Drive Parts - Top belt pulley clutch wheel for Toyota!...£9.50
Hand Wheel Drive Parts - Hand Wheel Singer 5800 range!...£23.50
Hand Wheel Drive Parts - Hand wheel clutch 9800 range!...£29.50