The window.history.pushState({"url":url}, "", url); call not only changed you URL, but also stored the state of the page in your browser. Your browser's back button will behave sensibly. Try going back and forward now. This causes an onpopstate event to be fired rather than an actual page load. We listen for that event and load the content asynchronously. This means that your browser's back and forward functions will be faster.
You may also have noticed a loading indicator. When a page is loaded asynchronously, the part of the page that we are replacing has a semi-transparent div stuck on top of it with a "Loading..." message on it. This shows the user that the page is doing something. Work could be done to add an animated gif spinner, or make the loading message elipsis change (e.g. "Loading" > "Loading." > "Loading.." > "Loading..." repeated).
Go on to "Three" for more.