The process is pretty simple. You can visit the site and build a button, which you then deploy on a website (some web skills required). You'll need to decide what you're giving away and what you want people to tweet. The service will assemble the code needed for the button and let you copy it. Operating the button is automatic. People click, tweet, and then they are directed to a download link.
At first I didn't really see a point. You can do this on your own, and it's based on a really old idea. In fact, Amazon runs exactly this kind of promotion every so often, only they programmed it in house.
But then I started looking at how it's implemented and now I see the value. Creators have been giving away content since forever, all in the hopes of getting more attention. With this new service there is now an explicit quid pro quo. Both parties get exactly what they wanted.
I've never actually liked the pushy "won't you tweet about us" after getting a freebie, so there's a good chance that I'll never click on one of these buttons. On the other hand, I can appreciate the value this service offers to creators. I've bookmarked the site. I give away my content everyday, so if I can find a way to get a little more attention for it, I will use it.