Popunder advertising is a format where an advert opens in the new browser window or tab behind the consumer’s active window. It’s designed to be less intrusive than pop-ups, appearing only as soon as the user has left or minimized the key site window, developing a more seamless browsing experience. https://squareblogs.net/leafbeast0/popunder-advertising-a-guide-to-maximizing-engagement-and-conversions