I've had a quick play with the beta. It is very much beta and only the core functionality is there at the moment and it's not particularly "dynamic" currently. You can package up your custom WordPress install (complete with database, custom theme, plugins etc.) then package it up with a script they have, upload it and the full WordPress install then runs on their servers. Then, you can manually stop your WordPress instance and click the generate button which builds out the static site and pushes it out to their CDN. By not very dynamic, I mean that if you have 10,000 posts and you add a new post, it'll build out 10,001 posts again when you build a new copy of the static site out.
That said, although it's clunky at present, it works and it's a really great concept – I can think of lots of application. As you mentioned, the added security layer is a huge bonus. All paths are written out identically to your WordPress install (
themes etc.), so it's easy to tell it's built out from a WordPress install but obviously your WordPress site would be sat on a completely different URL.
At present, you can download your static site and upload wherever your like but there's no automation or anything like that.
As I mentioned, I love the idea that content editors would access the WordPress install and then this would automatically get build out when they're done with their edits i.e. have a generate button in the toolbar on on the dashboard. That way, any edits all get pushed out together, kind of versioning the content.