Pop-under advertising is one of the most powerful methods in selling online products today. It was first started by ExitExchange® in year 1999. ExitExchange® is an AD Agency that also maintains the largest Pop-Under network on the Internet.
A pop-under is a new browser window beneath the active window. They are usually smaller in size and limited in features like menu bar, address bar, toolbar, etc. When you close or minimize your main browser window you can see pop-under.
Technically the only difference between a popup and pop-under window is that popup appears over the main browser window while pop-under behind it. But from the viewers point of view pop-up ads are hated all round by visitors to any site. People don't like to see them at all. The first thing they do when one pops-up is closing it. That's why the whole modern browsers have built in pop-up blockers. If viewers do not like to see them how can one expect any return from popup ads? So most websites that were using pop ups changed them to pop under ads. They are not so irritating like popup ads and can give you much better return.
As I said there is no technical difference between a popup and pop-under window except pop-under lies behind the main browser window. So the script code which can generate popup is also able to generate a pop-under.
The first two lines of the code actually generate a popup widow of my index page. Notice the last two lines of the code. The command 'window.open().blur();' get lost the focus to the popup and 'window.focus();' gets the main window under focus.
To see the pop-under demo you have to minimize the main browser window.
In the above example we just realize the theoretical implementation of generating pop-under window. But when you try to implement it in real world of advertising you have to remember some practical consideration.