What you mean by PopUnders Program ?

[NZ]

Pop unders are good compared to popups popups are very disturbing but you can't feel any difference in popunder it will just open under a site without disturbing visitors.

So you mean pop-ads run behind our original web page, then I wonder if it's visible to visitors? Of course it should be that's why they are paying, but I just like to know how it's visible.

It is visible by the user as soon as he close the current window. But you're right I see sometimes websites that open the popunder, and close it after few seconds. I guess they get paid for each view, this is why I would not choose popunder as an advertising solution, or I would need to pay for popup that no user will see !