Popunder is an ad format of pop traffic. Popunder or clickunder is a new tab that appears under the previous page. This type of traffic directs the user right to the landing page with the offer.
How can the user trigger this?
Whether or not you want the new page will appear under the previous one. Popunders and clickunders are less obsessive than popups in this regard. The user may not notice the new tab under the main one. He will look into its content a little bit later, at a more appropriate time. At this point, he is likely to make a conversion than when he is interrupted in the middle of surfing.