TL;DR - Mobify.js fires off one request to Jazzcat, which grabs all of your scripts and returns back a single concatenated file. As for how it actually works, we use the DOM pre-parsing capabilities of Mobify.js to grab script URLs and remove the scripts from the source document before it's rendered by the browser. THEN a request is made for the single javascript file from Jazzcat.
The feature currently only works for Mobify.js projects. Soon, you will be able to use this service on any website by adding a single line of Javascript into your existing site.