Popunder advertising can be a format where an advert opens in a very new browser window or tab behind the user’s active window. It’s designed to be less intrusive than pop-ups, appearing only following the user has left or minimized the main site window, developing a more seamless browsing experience. https://watts-warner.federatedjournals.com/popunder-advertising-a-guide-to-maximizing-engagement-and-conversions