How does autosave work? I don’t see any settings to adjust how often, and I don’t see it saving ever except for when I do a manual save.
My advice: turn this feature off, since it’s absolute nonsense in my opinion, even dangerous. In an environment like RapidWeaver this “feature” really is the worst what can happen to you. You can destroy your precious projects with this sooner than you notice it. Example: Open your project because you have an idea to improve s.th. on your site. Then change some things as you thought it would be nice (maybe you even have deleted one or more pages for this). Realize that it wasn’t such a good idea and close the project. Boom: project destroyed, since the version you just did close was automatically saved. Yes, you can get it back somehow, but this whole thing is completely counterproductive in a web-development in my opinion and should be turned off by default when installing RapidWeaver (maybe it is already, I don’t know). Just my 2 cents… 
So does autosave happen only when you quit and therefore you don’t have an option to save or not save? Is that how it works? And does Versions work when you do a manual save while working on a projects, and that creates a version?
Answer to your 1st question: yes. Horror, isn’t it? I can’t answer your 2nd question, but you would have to immediately duplicate your project after opening (and save it with another name) to not harm your original project. Hope that helps. 
Seems like Autosave and Versions should be separate options.
I just tried browsing Versions and restoring to an older Version. Holy cow… this is messed up! It certainly does not work. Anyone else use this feature? Does it work for you? When I restore I get a zillion other windows that open. Also, it locked up on me and I had to force restart. Bad all around.
No Bruce, I didn’t ever use this feature because I’ve been warned about it just in time. So, I think your experience does show that you should immediately turn off this nonsense-feature and simply just forget about it…
I never really used this option. RW crashed always.
I stopped autosave in the Mac OS too. I will decide whether I want to back up.
Sorry, the screenshots are in German 
I have this “feature” turned OFF as well. Not only because it can screw up your project, but also it slows your work to a crawl, when you have to repeatedly wait for a very slow saving process (that’s how saving works in RW, at least on my computer).
In my opinion, this “feature” is not thoroughly thought out by developers. To be clear, I have to say that auto-save is turned OFF system-wide on my computer. None of my applications uses it.