Popunder advertising can be a format where an advert opens inside a new browser window or tab behind the consumer’s active window. It’s meant to be less intrusive than pop-ups, appearing only following your user has left or minimized the primary site window, setting up a more seamless browsing experience. https://squareblogs.net/leafbeast0/popunder-advertising-a-guide-to-maximizing-engagement-and-conversions