I know that you are talking about piston engines, but as another example, I flew in a P-3 Orion for several years, with Constant Speed props (I was a Tube Rat - the pilots just drove us to work...).

Anyway, the engines were Allison turboprops and had only two speed settings - high and low. That's because the Turboprop (at least this particular engine) was efficient only in a narrow RPM range. The "Low" setting was used for taxiing and other ground work, the "High" speed was used for normal flight. The principle of changing speed was the same as has been stated earlier, to increase speed the throttles were adjusted to provide more fuel to the engine and the props increased pitch.... and vice versa.

Bob