When using direct link for skimmed traffic the publisher should apply ?bu= at the end of the tag (as long as this parameter is the first one he is using in this link)  and then add any back URL he wants to use there. 

For pop unders (when the direct link function has been activated in publisher's account) he should apply &bu= at the end of the link (in case there are already other parameters in the URLs)  and then apply any back URL he wants to send unsold traffic to