Because of the way an internal combustion engine breathes, it can produce torque, or twisting force, only within a certain speed range. Most modern engines have a narrow torque band of approximately 2500 to 4500 rpm.
To make use of the narrow torque band, a manual transaxle is employed. Through the use of gears inside the transaxle, the engine can be kept within its peak operating range.
The heart of the manual transmission is the gear cluster. Basically, two sets of gears (on two separate shafts) are placed side-by-side, permanently meshed together. Between the gears, synchronizers are used to connect the gears and direct the power from the engine to the wheels. The synchronizers are operated by shift forks, which are moved by the shift lever inside the vehicle.
On the majority of modern transmissions, the forward gears are synchronized, meaning that they are connected together with synchronizers, instead of the gears themselves actually moving. The one exception is reverse gear. In most cases, the reverse gear is usually a direct mesh, meaning that the gear itself is moved into position when the shift lever is placed into reverse. Since the reverse gear is not synchronized, a slight "clunk" noise may be heard when the transaxle is placed into reverse. If the reverse gear grinds excessively when placing the transaxle in reverse, the clutch is most likely not fully disengaging when the pedal is pushed.