Pozidrive - The control rod is a hydraulic piston with attachments as hoistop and nodrama say.
The input arm acts on the servo pilot valve to direct hyd fluid to one side or other of that piston, thus moving the control rod in and out.
The reason for having the other end of the input arm connected to the control rod is to provide the follow-up action and recentralise the servo valve when the TR pitch has been driven to the required position.