Basically, this is what makes it so great: Everything that you stick inside its brackets is ready to go at the earliest possible moment — as soon as the DOM is registered by the browser.
Waiting on the window is actually slower than waiting on the DOM.
I don't know that it's all that smart. It uses the DOMReady event in all modern browsers, and uses a load of tricks to get the same behaviour in older browsers. It will never fire before the DOM is ready, like the function in the article does.