I knew I'd need this one day
I used to be a Metro III system's instructor (it's been a few years!) but I still have a copy of my Metro Systems book.
<< Selecting the "AUTO" position provides electrical power to the NTS pressure switch through both poles of the ignition selector switch and the relaxed ignition mode relay. The engine mounted NTS pressure switch closes when negative torque (high oil pressure) is sensed, providing power to the ignition exciter. Once the pressure switch has closed, it will remain closed for approximately 30 seconds after the negative torque condition ceases.>>
So RPM has nothing to do with it.
But there is an NTS lock-out when the power levers are brought back for landing. Nobody needs two NTSing propellers when you're trying to flare.
I hope that answered your question.