Microsoft to use click-through as a search relevancy indicator

The other day Microsoft Research published an article on using click-through metrics to improve search results. While the article in a precise science manner proves effectiveness of the method, a stupid question remains open to me: how can a click-through tell about relevancy of the results, if user does not see a page behind a link, when clicking on a result?

Anyway for those enjoing maths here is the link to the article