Understanding the Manual transmission
Because of the way the internal combustion engine breathes, it can produce torque, or twisting force, only within a narrow speed range. Most modern overhead pushrod engines this is somewhere around 2,500 RPM (Revolutions Per Minute). By 4500 RPM they are producing so little torque that continued increase in engine speed produce no power increases. The torque peak on overhead cam shaft engines is generally much higher, but much narrower.
The manual transmission and clutch are employed to vary the relationship between engines speed and tire speed of the sheels so that adequate engine power can be produced under all circumstances. The clutch allows the engine torque to be applied to the transaxle input shaft gradually due to mechanical slippage. Consequently, the vehicle may be started smoothly from a full stop. -Chilton total car care