Sorry no can do on diagrams the CAMO says there is copy right issues.
Anyway the NTS system on our versions as I thought is a completely separate system to the torque sensing system. Its a ring which slips and actuates a valve which opens a bypass valve which dumps the oil pressure from the beta tube which then the spring pushes the blades to feather.
The torque sensor is a strain gauge bridge on most aircraft with a rare old hydraulic system on one of them.
And as confirmed this very evening that negative torque can be obtained by disking the props and then it goes back to +2 again and then when you apply power again it goes to what ever you want without the NTS triggering and without the blades going to Feather and the power levers forward not in beta. The engineers are sorting the flight idle out as we speak up a couple of notches to +6 and I will do a flight idle check tomorrow to ensure it doesn't dip to zero or negative.
Negative torque is when the propeller is driving the engine not the engine driving the prop.
Its a single action prop BTW
I really don't think they were anywhere near feathered it could be that the negative was when the AOA of the blade was taken to negative value and the blade drove the engine then as fuel was increased to the engine it started driving the blades again. Even if you put the power levers forward again the spring isn't meant to be strong enough to rotate the blades back into a flight AoA. It could be that he put the lever forward again which increased the power but as the blade couldn't get into positive AoA it just gave more reverse which would been seen as a positive torque as the engine was driving the blades not the blades driving the engine. I can't really see how anyone can work out what angle the blades were in. You can't even look at them after the event because without the start locks engaged they will just go to feather as the oil pressure drops off.
If the spring isn't strong enough I can't see how the NTS system can even feather the props because all it does is dump the oil pressure out the beta tube. So even with zero pressure in the beta tube if the forces on the blade stopping it rotating back to flight AoA's are enough to defeat the spring it won't feather.
When we power back off stand the torques read positive in reverse then decrease to zero then increase again as you apply forward movement to the power levers bringing them out of the rev range.