White Bear
Do you mean reduce power with throttle?
By reducing RPM with prop lever you increase the manifold pressure in a normally aspirated engine.
Its a good thing to do (reduce RPM) when you start a descent...especially above full throttle height to put more load on the engine. Other wise its like riding a bike down a steep slope and trying to peddle, your legs go flat out bouncing around the place and may cause damage to your muscles!
But of course as you loose more height you will need to reduce MAP to keep it around the desired cruise/descent setting.
regards 1M