One invitation is enough for me; thanks Chisel. KoK, what's a beanie, please?
So... Having established that we can think of prop pitch as the gears on your automobile, what about constant speed props? Well, for a start a C/S prop DOESN'T ACTUALLY turn at a single constant speed. What it does do, is maintain speed once you have set it (in AH by using the plus and minus keys) regardless of what you do with the throttle.
The relationship between prop pitch and prop rpm is simple but can be confusing. The less pitch we have the easier it is for the engine to turn the prop; the more pitch we have, the harder it is. {Zero pitch is when the prop blades are lying across the axis of the fuselage and doing no work. However, because they are 'edge on' the engine finds it dead easy to turn the prop. Max pitch (90 degrees) is when the blades are fully lined up with the fuselage. In the latter situation they aren't doing any useful work either, and as you can imagine, the engine would have to work its guts out just to turn the prop round - without achieving any forward thrust. but in between they are catching the air and shoving it backwards.)
Let's say you have 50% throttle applied, and you hit the minus key. The revs REDUCE because this applies MOARE pitich to the blades and as we haven't applied any more power to compensate, the engine and prop can only slow down. The opposite applies: hit plus and the revs INCREASE, 'cos you have just REDUCED pitch, making it easier for the engine to turn the prop.
Now,with a constant speed prop, if you set say 2000rpm on the prop tacho (rev counter), a gadget inside the propeller hub (the constant speed mechanism) will try to keep the engine and prop rpm at that speed. If you add power it will add pitch to absorb it: if you reduce power it will fine off (flatten) the pitch to compensate.
So, if engine and prop revs stay the same regardless of what you do with the throttle lever, how do you know how much power you have applied? That's where the manifold pressure gauge comes in.
At this stage, we needn't worry about what manifold pressure actually is,or how the engine supercharger maintains it at high altitude; just think of it as a power meter. Forget inches of mercury and all that: the lowest figure you can get is idle power, the maximum figure is full power. (Jets have their power meters calibrated in percentage to make it easy.)
So what prop rpm figure do we set when? Coming back to the automobile analogy, where fine pitch (max rpm, remember) equals low gear, use that for when you are taking off, landing, or want to accelerate from a low cruising speed. In the cruise, reduce the rpm to minimum, cos that gives the most efficient fuel consumption and then set the airspeed you want with the throttle.
HOWEVER, I understand that the effects of changing from hi to lo prop rpm are not yet modelled in AH, (SOMEONE PLEASE CORRECT ME IF I'M WRONG} so just set the prop rpm to max (fine pitch) and leave it there.
Please let me know if this has been helpful. If you want a more detailed explanation, including how the constant speed mechanism works and why it can only control rpm within certain limits, read 'Those marvellous props' by John Deakin at
www.avweb.com/articles/pelperch. 
