To be able to drive a primary control circuit, the AP uses what is in effect a bi-directional cable winch that allows a small electric motor to apply the normal forces a pilot does. Pretty much all conventional cable operated flight control systems use that kind of A/P servo. So you can say that the A/P is driving the control surface directly, just like the pilot, but the A/P servo is a fairly "weak" pilot in that it's maximum force is less than a human pilot can generate, only what is needed for normal control.
The servo's electric motor drives a winch capstan drum through a reduction gearbox, and the winch cables are tied in somehow to the cable circuit itself to be able to pull on either side of the cable circuit, or into a bellcrank in the circuit when the motor is energized in one direction or the other.
There are two clutches between the motor and the capstan drum; an on-off tooth clutch and a slip clutch. The tooth clutch is two discs with radial gear teeth around the perimeter that intermesh when the discs are brought together. This clutch is operated by the A/P computer when the A/P is engaged/disengaged by the pilot (or disengaged by the A/P computer for various reasons), and is either fully engaged or not engaged. They are normally spring loaded disengaged and have to be energized by solenoid to be engaged, so they are normally fail-passive electrically speaking.
But just in case they do jam together for some reason, just upstream or downstream of the tooth clutch is a passive slip clutch that is there in case the tooth clutch can't disengage, so that the control circuit is not completely immobilized by a dead servo that can't disconnect (it also limits the force the servo can apply).
You will be able to move the controls by forcing the clutch to slip, but the effort is very high and it is quite unpleasant to have to control the airplane with controls that feel like they are set in partially cured concrete, but at least you can move them.