A vehicle's transmission is what converts power made from the engine to movement in the ;wheels. Transmissions allow a car to move forward from a standstill or in reverse. Having different gears allow the car to move at various speeds and allows the car engine to continue to run while the vehicle is stopped. Automatic transmissions use hydraulic coupling to shift between gears unlike manual transmissions which rely on the driver to shift between gears. Hydraulic coupling is what is between between the engine and the gears. Since the transmission relies on this hydraulic coupling for shifting, the fluid is very complex. The fluid also has to act as a cooling agent for the torque converter assembly and lubricate the gears in the transmission. ;