I don't know how it's done in the sim, but i guess that it should in reality. Less propeller RPM means less engine RPM as well (since they are connected to each other), which effectively means less combustion cycles per minute.
I had a lengthy post about these things recently. It's encouraging to see people want to learn the complex settings, but that post was a wall of text and i can't retype everything without forgetting something. I repeatedly proof-read that post and now that i look at it it's obvious i still made some typos/mistakes due to its length. I'll see if i can dig it up for you and give you a link.
Edit: Got it! Take a look here, there's also useful input by other forum members, some with attached pdf documents as well:
http://forum.1cpublishing.eu/showthread.php?t=17139