The direction that the motor runs is controlled by the cockpit clutch switch. In the engaged (flight) position, the motor will only run forward, increasing the tension on the belts. Thus it will never decrease tension on the belts in flight.
If the belt tension sensor fails, the clutch system may run the motor in an attempt to increase what it believes is a low belt tension condition. In this case, there is a real danger that the belts will be tightened so much that they will break.
The POH calls for the pilot to monitor the automatic clutch system while in flight, and if he sees the motor run for more than 7 or 8 seconds, to pull the clutch system circuit breaker. This will cause the motor to stop running, and the upper sheave will stop in its current position.