RoboHelp Publishing

I just stumbled across this discussion because I'm having trouble with RoboHelp wanting to republish every topic even if I only change one, so the time required to publish is long even for simple changes.  There's a statement here that RoboHelp only updates changed items by default, but it seems like the first thing that happens when I generate WebHelp is that RH clears the output folder and starts a fresh copy. How exactly does RoboHelp decide what to publish, and is there a definitive way to make sure it only uploads modified topics?