I think B2N2 is a little wrong about turning the prop over against compression causes highest drag.
It takes more energy to turn over with a closed throttle, as pulling a vacuum, with it fully open the pistons bounce on the cylinder pressure, which acts like a spring and recovers the compression energy. So fully open and full course is the lowest drag for a spinning prop. Of course full coarse for a stopped prop if achievable closest to feathered.