Hi there
GroundBound,
First the caveats: I converted from Piper Warrior to VP version of it, the Piper Dakota (PA28-236) earlier this year, so I think I can answer you in parctical terms - albeit with only recent experience. No doubt someone more experienced than me can make a better technical explanation, but here is how I see it:
The VP mechanism is a governor, usually driven by engine oil pressure, which basically varies the pitch of the prop and senses the resultant speed, aiming to keep it constant. Its an analogue feedback loop, in engineering terms.
The throttle works the same as before - more power = go faster! But because the governor is keepin the prop at a constant speed, the RPM indication doesn't change thereffore doesn't help if you want to set a precise setting. Thats why we have the additional Manifold Pressure gauge in VP aircraft.
On a VP aircraft, the MP gauge provides an indication of "how much throttle" you have selected. So, 24" MP might be the cruise setting, 20" might be selected for decent, and 17" for the circuit say. If it were fitted to fixed pitch prop a/.c it would do the same, but is unnecessary.
You have the prop lever adjustment as well in order to set various prop. speed settings. The a/c manual (POH) will have a table (also printed behind the sun visor on the Dakota) with recommended combinations of MP/RPM settings for various presure altitudes and speeds to set a given % power. Some settings you commit to memory for the type you are flying, e.g. maybe "24/24" for the cruise - which would be 24" MP and 2400 RPM. On the Dak at our club, we use "24/22", i.e. 24"MP and 2200 RPM, as it gives a bit less noise for the neighbours and prolongs engine life a bit.
Its not as difficult as it sounds. I do remember finding it awfully confusing when reading about it - but once flown using VP, it all becomes clear and is not really difficult. I recommend you get some conversion training with a friendly instructor on a VP type, and then you'll see for yourself.
Most of all, what I like about VP props is that the a/c performs better - faster ROC, faster cruise
Hope this helps,
Andy