The clutch uses friction between two plates to couple the transmission to the movement generated by the engine. Without a mechanism to control this coupling, the transmission and engine would have to stay connected. Braking would then require slowing and stopping the engine itself, and shifting would crunch gears. The clutch, however, allows the transmission to smoothly engage the engine without causing it to stall.
A car's clutch mechanism includes the engine's flywheel and the transmission's clutch plate. The flywheel connects the engine to the starter motor at ignition, but its main function is to transfer torque, or rotating force, from the engine to the transmission. To do this, the flywheel is physically contacted by the clutch plate, which locks the engine and transmission input shaft and causes everything to spin at the same rate. When the clutch pedal is pressed, however, the clutch plate is released from the flywheel.
To protect the two discs and improve contact, the clutch plate is covered with frictional material. This layer should only wear when the clutch plate slides against the flywheel before and after it engages. Thus, drivers should take care not to slip the clutch frequently, as it will wear faster.