The fly article is very interesting.
however I think you mean in turboprops where negative torque is where the prop is driving the engine rather than the other way round. This can be due to high airspeed with a low engine power setting or an engine failure. The system senses negative torque and changes the blade angle (coarsens) to alleviate the situation. Also acts as a backup to Auto feather although it tends to have a more limited blade angle range to work in.
On the right track?