My guess...
The metros that I have flown all had the later version of the two ignition systems available, where you had OFF, which you could start in, but have no 'NTSing' feature (not normally used), normal flight was in AUTO ignition mode, and CONTinuous ignition was available for icing or turbulent conditions at pilot discretion - and also comes on when an 'NTSing' condition was present.
The system would not come on when there was only Negative Torque sensed, but there had to be less than FLT IDLE rpm on the prop, so depending how well its rigged, less than 70%.
If anyone else has other ideas, would love to hear them too.
ABC