torque converter

is is a modified form of fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. Like a fluid coupling, the torque converter normally takes the place of a mechanical clutch, allowing the load to be separated from the power source. Unlike a fluid coupling, however, a torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear.

The impeller drive the turbine and transmission.  Fluid flow is controlled by the stator.

Also see clutch, transmission, and planetary gears.