We laid out this steering rack housing and steering rack to illustrate the fundamentals of how the power steering applies force. Note that on the rack there is a fence or piston just a little left of center. On the rack housing you can see the hydraulic lines entering the housing on either side of where the fence on the rack rides. The other end of the hydraulic lines run to the input shaft (pinion) area of the housing, where the spool valve lives. As you turn the steering wheel, the spool valve ports hydraulic pressure to one side or the other of the rack fence, thus pushing on the rack.