A variation of pop-ups is pop-under advertising, which opens a new browser window hidden under the active window.
Pop-under (picture 1) does not interrupt the user immediately and is not seen until the active window is closed, making it difficult to determine the site that opened it.
If you are not very careful or if you do not have an adblocker installed you will not be able to realize that this type of ad has been opened.
That’s why this type of advertising is one of the most effective. Many advertising networks nowadays prefer to turn to pop-unders just because they do not mind the user. However, the pop-under shows your business or product but can not guarantee that the window with the open landing page will be accessed. This is great if you want just to promote your business/ page and to make it known all over the world.
It is very often used in combination with CPM = Cost per mile / Cost per impressions.
Pop-under can have many dimensions depending on where you want it to appear.