Just to add $0.02 to the prop governor discussion:
The governor is designed to this paradigm: To increase RPM, reduce blade pitch angle (i.e. reduce AOA); to decrease RPM, increase pitch (incr. AOA).
As long as the engine is producing positive torque, this paradigm applies nicely.
But if the engine ISN'T producing torque but is instead absorbing torque from the windmilling prop, the above paradigm creates bad results. As the RPM initially drops, the governor moves blades to a lower pitch angle, which now means a NEGATIVE AOA.
The prop now speeds up, and probably passes into a mild overspeed. The governor senses this, and reduces pitch angle some more. This of course means a MORE NEGATIVE AOA, windmilling the prop to an increased overspeed. This cycle repeats until the blade pitch reaches some mechanical stop. It's supposed to be a pitch-lock mechanism, which hopefully contains the problem before something breaks.