The Rudder Trim Actuator is electrically powered and repositions the Feel and centering unit to a new 'null' position. It is located under the flightdeck floor and anchors the bus rods that tie both sets of rudder pedals together. When you input some trim from the switch in the flight deck the trim actuator either extends or retracts and repositions the feel and centering unit - this moves the Rudder through the normal controls system to a new neutral point.