The RPM lever is the prop lever. The
throttle only controls the butterfly in the carb.
So with the throttle at idle, if you
push the RPM lever (prop lever) forward you are forcing the blades to go to the
full fine pitch position - decreasing the blade angle (pitch)
At idle that presents a big flat face
to the air so a lot of drag, so rate of descent will increase.
With the throttle open then it would
have minimum drag therefore maximum RPM.