A clutch disc is the part of a vehicle's clutch system responsible for engaging or disengaging the engine from the transmission. A clutch disc is sandwiched between the flywheel on the back of the engine and the input shaft on a manual transmission, which lets gear changes happen without killing the engine when the driver stops. Friction and Its Secret Sauce Technically advanced clutch discs (this Ford Focus item is from Sachs Advanced Clutch Technology) typically use multiple types of friction material along with a multi-finged or splined spring system to aid even wear and heat distribution over broad areas for longer service life. Fundamentally as a part of functioning for manual transmissions, it is an interface for connection to the engine enabling power delivery to wheels and once engaged, transmits rotational power.