Basically we intercept your click, call
pushState() to change the browser's URL, load in data with Ajax, then slide over to it.
When you hit the back button, an
onpopstate handler is fired after the URL changes, making it easy to send you "back".
Want more? Check out the HTML History API Demo and MDC's Manipulating the browser history documentation. Facebook has blogged about their use of this stuff, and Flickr has been doing it for months on their lightbox view.
We're still getting all the kinks out of the Tree Slider, but we hope you like it!