This may not be the asnwer you're looking for, but for me it seems that it depends on the plane. For the Alphasim C-7, you just bring the prop (RPM) controls all the way back, at which point the props are in reverse pitch. Then any increase in power makes you go backwards. However, this does not work in the Aerosoft DHC-6 Twin Otter. For that plane, the only way I can go into reverse is to hold the F2 key down. The F2 key is "decrease power quickly", which it does, but then comes up again, with power in the other direction. Wierd. Then if you touch the throttles, it comes out of "reverse mode" and all that power is now instantly being delivered forward. I'm sure others here will have the correct answer!