109 prop pitch (rpm) and the supercharger

[Crumpp]

"feather" prop to add rpm

You would not feather the prop to add rpm. Feathering is where you coarsen the blade pitch to the point the chord is parallel to the relative wind.

A feathered propeller produces no thrust at all and minimal drag. It would be very poor practice to run the engine with a feathered propeller and the engine is shut down when the propeller is feathered.

It is done to keep the shut down engine propeller from windmilling and causing loss of control.

http://www.youtube.com/watch?v=E7x8iAjFtzI

You are right about the Bf-109 pilots doing the aerial equivilent of "popping the clutch". That is exactly what they are doing so to speak. It is an advantage of a controllable pitch propeller. It is something that would take experience to do. The pilot must understand the principles of propeller operation regarding pitch and the relationship of manifold pressure and rpm.