The clutch is an essential part of manual transmission systems.The hidden parts include a clutch plate,typically of "8" to "10" diameter,with friction material on both faces and splined to the gearbox input shaft.The clutch diaphragm is bolted to the engine flywheel;it is a  very strong spring which can clamp the clutch plate to the engine flywheel and force it and thus the gearbox input shaft to rotate with the engine.A clutch disengagement mechanism can release the diaphragm and allow the clutch plate and the engine to rotate independently for the purpose of changing gear.