Basically, it's going to throw any debris towards the center of the birds body instead of throwing it into the front and rear (Camera, etc.) of the bird. Some folks believe that it flies better but I haven't seen any factual evidence of that yet. Personally, I'd rather clean up my camera and the rear of my bird rather than having things thrown into my stack or electronic components.
Reversing your props supposedly also helps your props get cleaner air (less turbulent). I changed mine a couple weeks back, was easy with rf1. Havn't noticed a huge difference.