When the rack-and-pinion is in a power-steering system, the rack has a slightly different design.Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist.
There are a couple of key components in in addition to the rack-and-pinion or recirculating-ball mechanism.