Err, I think I hear the sound of a fresh can of worms being opened here.
Please don't think of VP props as 'like gears on a car' because they are not.
The reason for having a VP prop (not all are constant speed) is to use the power produced by an engine as efficiently as possible; ergo, the RPM lever (for a constant speed prop) is a 'command lever' for the engine speed.
If high thrust is required then a high RPM is selected to realise that thrust and the govenor will regulate the pitch to achieve that RPM. Statically, this will result in the prop being quite close to it's fine pitch stops but will progressively move courser as the aircraft accelerates.
An engine will only develop it's rated horsepower when turning at Redline RPM, therefore in a fixed pitch machine at take off the amount of power available will typically be less than full HP. Static thrust is equivalent to the cube of the RPM therefore you will be missing out on a great deal more thrust than just plain HP.
When cruising the POH will provide values for RPM and manifold pressure (MP is NOT a measure of throttle opening but is roughly equivalent to 'load') which will provide a good balance between engine efficiency and propellor efficiency. Since drag increases to the square of the speed but thrust reduces in a cubic relationship the aerofoil of the prop must reside at a happy medium between thrashing around creating too much drag and turning so slowly as to produce too little power.
The constant speed bit is exactly as described - a governor for making life easier and more comfortable. There are two main arrangements. One uses counterweights and the other uses a dual acting oleopiston in the propellor hub. Both essentially do the same thing by 'sensing' an over or underspeed condition as it relates to the lever position and correcting the blade angle to compensate.
It's all a bit long winded but I am available for full CS/VP type conversions if interested.
I use the maxim 'Fine First' - i.e. a power setting requiring an increased RPM should start with the RPM lever being advanced first and vv.
Cheers all