Most of this has been reviewed in the thread elsewhere, but to perhaps summarize from a B737
-400 AMM describing the HS trim system:
The horizontal stabilizer trim control system provides longitudinal trim of the airplane by varying the angle of attack of the horizontal stabilizer. The horizontal stabilizer is moved through 17 degrees of travel by means of a jackscrew with ball nut. The main electric actuator and the cable drum on the jackscrew gearbox provide for trim control from three separate control systems. The normal control is an electrical system which actuates the jackscrew through the main electrical actuator. The main electric actuator and autopilot actuator, autopilot control is provided by the autopilot actuator on the gearbox. A manual control system drives through cables to the cable drum on the jackscrew gearbox. The manual system remains engaged at all times and is therefore back-driven by the main electric actuator or autopilot actuator during normal operation. Manual system operation will disengage both the normal electrical and autopilot actuators if these systems become jammed. A continuous indication of stabilizer position is provided by trim position indicators adjacent to trim wheels on the control stand. The indicators are positioned by the manual system. A takeoff warning system indicates any unsafe stabilizer position for takeoff.The control column actuated cutout switch located under the cockpit floor, stops electric trimming of the stabilizer when opposed by the motion of the elevator control.
Normal electrical trim control system consists of control switches, cutout switches, a column- actuated cutout switch, trim control relays, flap switch and relay, limit switches and an electric actuator. The electric actuator contains an electric motor, two electromagnetic clutches, a speed change relay, torque limiting clutch and output shaft to drive the jackscrew gearbox. The motor is operated by 3 phase AC power and the relay and clutches by DC power. The stabilizer actuator is controlled by the trim control switches, control cutout switch, limit switches and cutout switches. The control switches, located on the outboard horn of each control wheel, have two momentary positions, nose up and nose down, and are spring-returned to the center off position.The column- cutout switch located beneath the cockpit floor is actuated by forward and aft movement of either control column. Stabilizer limit switches are located on the bulkhead aft of the jackscrew gearbox and actuated by a striker to limit stabilizer leading edge up and down travel. The cutout switches on the control stand are used to remove power from the main electric actuator or the autopilot actuator.
Normal electrical trim control system consists of control switches, cutout switches, a column- actuated cutout switch, trim control relays, flap switch and relay, limit switches and an electric actuator. The PSTA contains an electric motor, two electromagnetic clutches, a speed change relay, torque limiting clutch and output shaft to drive the jackscrew gearbox. The motor is operated by 3 phase AC power and the relay and clutches by DC power. The PSTA is controlled by the trim control switches, control cutout switch, limit switches and cutout switches. The control switches, located on the outboard horn of each control wheel, have two momentary positions, nose up and nose down, and are spring-returned to the center off position. The column-cutout switch located beneath the cockpit floor is actuated by forward and aft movement of either control column. Stabilizer limit switches are located on the bulkhead aft of the jackscrew gearbox and actuated by a striker to limit stabilizer leading edge up and down travel. The cutout switches on the control stand are used to remove power from the PSTA.
Normal electric trimming of the stabilizer is done at one of two rates as controlled by flap position. Trim rate with flaps retracted is 1/3 the trim rate with flaps extended. The autopilot actuator also trims at one of two rates as controlled by flap position. High speed autopilot rate is equal to the normal electric low speed rate. The low speed autopilot rate is 1/2 the rate of the high speed autopilot rate.