The manual for the SFIM AFCS 85T31 states:
Whenever the series actuator nears saturation, the parallel trim actuator with full authority moves the stick at a slow rate so as to displace the anchoring point of the cyclic stick and maintain the series actuator around its mid point, thus restoring full authority
It also states:
When illuminated, the amber TRIM warning annunciator indicates:
1. A trim actuator runaway.
2. A trim computer failure.
3. A "Trim Demand" This is triggered whenever a P or R series actuator is out of centre by more than 2/3 of the actuator's stroke. This might happen in the case of an inoperative Auto-Trim
Note: When I converted onto the 355 F & N the helicopters I trained on had no autopilot fitted (VFR only), hence A/P theory and operation was not covered during the groundschool. The one I now fly has a full A/P system.