So now that we have a basic idea of how gearboxes works there's a second item in transmissions that one needs to understand - the clutch. The clutch is what enables you to change gears, and keep the car idling at one point without switching off the engine. In the absence of the clutch the engine will have to be switched off every time we bring the vehicle to a stop. The clutch is needed because the engine is running all the time which means the crank is spinning all the time. We need some way to disconnect this constantly-spinning crank from the gearbox, both to allow us to stand still as well as to allow us to change gears. This is where the clutch comes in.
The clutch is composed of three basic elements;
1.The flywheel,
2.The pressure plate and
3.The clutch plate(s).
In an automobile clutch, the flywheel is connected to the engine, and the clutch plate is connected to the transmission. In normal running they are in contact with each other and thereby transmit the power. The clutch cover is bolted to the flywheel so it turns with the flywheel. The Diaphragm spring is attached to the clutch cover housing using a pivot mechanism.
The ends of the diaphragm springs are hooked under the lip of the pressure plate. So as the engine turns, the flywheel, clutch cover, diaphragm springs and pressure plate are all spinning together. The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing.
Case 1) When the clutch pedal is pressed.
The clutch pedal is connected either mechanically or hydraulically to a fork mechanism which loops around the throw-out bearing. When the clutch pedal is pressed, a cable or hydraulic piston pushes on the release fork, which presses the throw-out bearing against the middle of the diaphragm spring. As the middle of the diaphragm spring is pushed in, a series of pins near the outside of the spring cause the spring to pull the pressure plate away from the clutch disc. This release of pressure allows the clutch plates to disengage from the flywheel. Now the clutch plates are disengaged from the flywheel and this permits the vehicle to be stationary while the engine is running. The flywheel keeps spinning on the end of the engine crank but it no longer drives the gearbox because the clutch plates aren't pressed up against it
. 
Case 2) When the clutch pedal is released.
When the clutch pedal is released the exact opposite happens. As you start to release the clutch pedal, pressure is released on the throw-out bearing and the diaphragm springs begin to push the pressure plate back against the back of the clutch plates, in turn pushing them against the flywheel again. Springs inside the clutch plate absorb the initial shock of the clutch touching the flywheel and as you take your foot off the clutch pedal completely, the clutch is firmly pressed against it. The friction material on the clutch plate is what grips the back of the flywheel and causes the input shaft of the gearbox to spin at the same speed.
The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel, and how much force the spring puts on the pressure plate.
