On the 124 cars a sensor is on the rear of the compressor that senses compressor rotation. If lack of rotation is sensed, the clutch circuit is interrupted. Turning the igniton off and back on resets it.

The idea is to shutdown a failed compressor before it scatters metal throughout the system. Several things besides an internally failed compressor can cause the sensor to trip; a slipping belt, a slipping clutch or a worn compressor that allows the compressor innards to have excessive thrust.

Hope this helps.