When you lift your foot off the accelerator in your car it will slow down much quicker (engine braking) than if you pushed in the clutch (free wheeling). Engine braking happens until the car slows to the point where the engine speed equals the road speed it would produce. Thus if the prop could free wheel at the speed required to equal the forward speed of the plane it would not act as a brake. If it were slowed more than that by a throttled down engine, it would act as a brake. A simple explanation and it does not include losses due to all the other aerodynamics but there it is.