The functions of a clutch are best described by the basic functions of the engine and the wheels of a running car: the engine is required to be in constant motion, while the wheels are required to stop and start upon request. Pressing the clutch pedal interrupts the transmission of the engine power to allow the driver to change gears and control the motion of the car. When the clutch pedal is not pressed, the clutch is engaged and there is constant pressure on the clutch plate via the diaphragm spring. This constant force and heavy friction means that clutch system can wear down over time and can break under the pressure, by which point a complete replacement is required.