italia:
What you're talking about is engine inertia which only produces a turning moment
No I really don't think that is what morrisman means. He is talking about the fact that if the RPM is stable, then that is because thrust horsepower equals brake horsepower at that moment in time.
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tcyandy:
What I would add to the answers above is that the two torques are different due to the presence of the reduction gearbox. The slower turning shaft - ie the prop - will have a higher torque than on the engine side. The power will be the same though as the relationship is: power = torque x rpm
As an aside, this relationship of shaft speed, torque and power transmitted is illustrated by the spindly drive shafts you can see on the tail boom of a helo. The rpms are stepped up to reduce torque and enable a smaller shaft diameter, then reduced again at the tail assembly.