Cybetech - a SAS will need sensors (usually gyros) and linear actuators in the control runs to do its job.
The linear actuators are fast acting but with limited authority (typically 10%) so that a runaway can't produce a 'hard-over' condition.
The actuators are just a motor and a screw arrangement that can either shorten or lengthen the control runs between the flying controls and the main jacks.
A rapid control movement or disturbance (a gust of wind for example) will alter the aircraft attitude, be sensed in rate by the gyro which sends a signal to the SAS computer which opposes the movement using the linear actuator in the appropriate control run.