sam92,
The previous 2 posters have passed on very good information to you. It's extremely valuable to remember and understand, as, following engine failure in a single, the propeller RPM control can be used to vary drag to adjust descent profile during the glide to the forced landing. (High RPM = More Drag, Low RPM = Less Drag).
The rate of descent (with idle power) change between High and Low pitch is VERY noticeable.
I hope that you don't need to use it in anger, a possible life-saver if you do
Regards,
Old Smokey