It isn't one specific url i'm reffering to, it's more the site. But you really have to register for those sites, to see it in action.
The thing is, on you register as a user, then you get points for ordering items, signing up for newsletters or contest where you can win an ipod. Then you go and register for one of those, which is provided by either or, and when you have pressed the submit button on the external website, registers that, and give you your points for signing up.
I hope this will shed light on the question.