This is clever and I can tell you how this works.
1.) There is an "input server" running on the PC that hosts a web service.
3.) The web page sends the data to the web server running on the host PC via an AJAX call.
4.) The PC translates the HTTP request into a button press.
5.) Software receives button presses as input.
When I clicked on this article, I was hoping to see PC based software running on the tablet controller. That would have been epic. I doubt it would be possible to encode and stream PC images to the controller through the web interface fast enough to do this though.