I think this needs some more explanatory text. It's a nice quick mockup generator, it would appear, but it oddly exports not HTML, but rather a nodeJS application, which seems insanely heavyweight for a simple form builder.
which doesn't help much, but it does seem to have an overview document that helps somewhat:
https://github.com/mozilla/napkin/blob/master/docs/overview....
It looks like links and forms can be linked inside the mockup generator, and it's that linking that they are trying to preserve by exporting a node.js app. I'm not sure that I'd care enough, to lock things in in this fashion, but perhaps it's a nice tool if you are already planing an app using Node/Express