If you are familiar with the design of vehicle water pumps, then you already know what an impeller is. The impeller on a water pump is a round component with vanes that turns on a shaft. When the engine is running, the spinning impeller vanes force coolant to circulate through the coolant passages and radiator.

The impeller vanes on a torque converter work in a similar way. The spinning impeller forces hydraulic fluid to circulate via centrifugal force. The fluid is carried in a circular motion by the vanes, and, as speed increases, the fluid flows away from the center of the impeller.

As the fluid flows outward, the vanes carry it toward the upper edge of the impeller. As impeller speed increases, the fluid gains enough momentum to flow off the edges of the vanes and out of the impeller. The fluid comes out of the impeller with enough force to drive the transmission input shaft if the force is properly directed.