Ah OK, so I was in fact wrong. The motors, or the cockpit trim wheels
do open the hydraulic valves, which allow pressurised fluid to drive the THS hydraulic motors to move in the appropriate direction. As the THS moves, the mechanism connected to it starts to close the hydraulic valves again, and the THS stops when the valves are fully closed, at which point it has reached the trim position commanded by the motors/trim wheel.
Can be imagined as a lever opening or closing the hydraulic valves, with one end of the lever moved by the motors, and the other end moved by the THS. Say when the lever is horizontal, the valves are closed. Moving the lever up or down via the motors or the cockpit trim wheels opens the valves, which moves the THS, which runs until the lever is horizontal again, at which point the valves are fully closed, so movement stops at the new trim position.
Got it
PS, this also explains how the THS can still be operated, (manually), in the event of complete electrical failure, as long as Green or Yellow HYD is available.