It works like this: you have some links somewhere inside the DOM, and inspecting the CSS "visited" attribute, you can find out if the user's history contains the URL you're linking. Of course it doesn't actually have to be visible to the user.
His nice white-hat example presents only appropriate "add to your <social app>" links to readers. I can think of some less nice examples. You're selling stuff and want to know if the person is a customer of your competitor. Check to see if they've visited the logged-in area of the competitor's site, then present a specific comparison between your competitor and your own products. Or you could be very reactive to stuff being written about you in the press in the press, even though the person read the article a few days ago. Cool!
I'm looking at personalisation at the moment as a way to try and mediate around the inevitable conflict of home page real estate between divisions in the company. The more we can work out about people hitting our site, the more accurate we can be. This gives the evil genius in me some ideas! Mwahahahaha!!!