Well, the easy answer is that you're changing the pitch on your prop. When prop pitch is increased (called coarse pitch) it takes a bigger "bite" of the air which increases the load causing it to slow down. The reverse happens when pitch is decreased (called fine pitch).
AH lets you set a specific RPM that you want to to maintain and the prop governor will automaticially vary the pitch angle to maintain that RPM (within engine and airspeed limits of course). Why would you do this? There are two big reasons. First, props are airfoils and, just like wings, they work most efficiently within a certain range of angles of attack. The prop AoA is determined by pitch and airspeed. Second, engines also have an optimum RPM and air/fuel mixture for producing power, minimizing fuel usage, and keeping operating temps low so you want to operate the engine at its best RPM for what you're doing. The variable pitch prop lets you match optimum engine RPM and prop pitch for best performance whether it be power, fuel economy or engine life. For instance, there is a combination of RPM and prop pitch which is best for climb and a different combination best for speed and still another for best range.
I mentioned above that "AH lets you set a specific RPM". All AH aircraft use the same type of prop control called Constant Speed, i.e., it'll hold a specific RPM. But, RL is a bit more complicated as all WWII aircraft did not have the same type of prop control, some were simply fixed pitch props with no controls at all. Others had adjustable props which could be set by mechanics on the ground and some were adjustible airborne but the pilot was limited to setting a specific pitch vice RPM. Others, particularly late war aircraft, had actual CS props. HTC makes them all the same to simplify things for the pilot in the same way that we have automatic mixture, boost control, and fuel management.
