Sterling Power Alternator Protection Device 12v
The APD works when the spike back voltage to the alternator shoots above 16.5V -> infinity (potentially) as the current has gone to 0A (to conserve P = I x V).
The APD turns on with a small resistive load of mA to reduce this escalation of voltage as a result of the break of cable that has led to 0A and V goes up up up.
If the spike is excessively rough, it can lead to APD damage (which can’t be fixed) but, your alternator / regulator has been protected.
Owner’s Manual and installation instructions for the Sterling Power Alternator Protection Device:
http://www.sterling-power-usa.com/library/Sterling Power Alternator Protection Device.pdf