I am a guy doing B.Tec in Engineering specialized in Automobile and i think i can explane to you what the function clutch is, in an automobile or bike.
The clutch in a manual transmission system works mainly by foot control (driver's foot). The driver, of a car with a manual transmission system, operates the clutch by pressing his foot down on the clutch pedal. Now the clutch pedal, which is in turn attached to the clutch disk by a cable, is the factor responsible for engaging and disengaging the clutch disk from the driveshaft and transmission. Now, the clutch is what allows the manual transmission system to function with ease. Whenever there occurs a need to shift a gear in a manual transmission system, the clutch pedal has to be fully pressed, which in turn engages the clutch through the clutch cable. For a manual transmission, the clutch mechanism is an absolute necessary. When the clutch is pressed (engaged), it in turn disengages the crankshaft from the transmission of power to the axle, which in turn prevents transmission operation from happening.
On most motorcycles and super bikes, the clutch is operated by the clutch lever located on the left handlebar. Racing motorcycles use, "Slipper Clutches" to find out the effects of braking of the engine which, when applied only to the rear wheel, can cause or may lead to instability.