This fraud method uses "Background Image" setting within a CSS file, where the source of the image is set pointing to the Click-tracking URL. The broweser request will also recieve a cookie from the tracking server, The browser stores the cookie on client computer. Subsequently, when the visitor makes a purchase at the advertisers e-commerce site, the cookie will identify the rogue affiliate as the source of the traffic, and the rogue affiliate will recieve the commission.
It is usefule for the tracking software to examine the "Referer" field in the header, to ascertain the source of the click.