The pump consists of the flow-control/piston assembly and the pump itself. From right to left is the flow control, the piston or poppet, and the piston spring. Consider this the door that the pressurized fluid opens and leaves the pump through. The piston floats within the chamber in case the backpressure overwhelms the piston. Backpressure occurs when the steering is turned to full lock and then pulled hard until the pump buzzes. This backpressure is bad for your pump because it recirculates and cooks the fluid.