How does the github UI navigate directories without postbacks?

When navigating through my repo directories on github.com, I noticed that it uses cool javascript effects to switch between directory views without ever causing a postback, and yet the url changes nice and RESTfully: http://../tree/master/dir7 -> http://../tree/master/dir7/dir4.

Whenever I have to keep track of navigation with javascript, I've always had to resort to hash arguments in the address bar. http://...page#new-javascript-location

How do they do it?