"Feathering" actually happens to the prop even though, as you say, the phrase is often used in conjunction with the engine itself. In short, when an engine fails and is shut-down, the pitch of the prop is changed so that the leading edge of the prop blade is facing directly into the airsteam. By doing this the prop is not forced to "windmill" ie blown round by the airflow. This reduces drag from the dead engine and also reduces the chance of damage to the reduction gear and the engine itself.
As an aside, when turboprops are shutdown on the ground, the props are frequently feathered - applies a braking effect to the prop and also stops it being blown around by the wind. Turboprops are fairly freewheeling since there is none of the cylinder compression resistance that you would get from a piston engine.
hope this helps - sorry, it was longer than anticipated!
Good luck with the Shack plans - one of my favourites (Dad did a tour on them in Malta back in the '50s) Let me know how you get on.
cheers
Rick