We replace all of the links in your email with a redirect. When a recipient clicks one of these links, they will hit our servers for a fraction of a second before being redirected to the intended destination. They probably won't even notice this rerouting. We record the hit on our servers, as well as the merge tag that was included in the URL. In this way we can track not only how many clicks a link got, but also who clicked on the links.
We then use this data to generate a heatmap of which links got the most clicks.