Pop-under windows, as their name suggests, open underneath the front window. The idea behind using pop-unders is that in many cases the window will open in the background without the visitor even noticing.
The great advantage there is that the pop-under won't get closed early before it has finished loading. Not having to worry about creating a page that loads quickly, you can safely put more content in the page. It will sit there waiting to be viewed until the front window is closed, or until (Windows users) click on it in the Task Bar - even if that's an hour or two later.
Pop-under ads make the whole thing a softer sell. The visitor may have long since left your site by the time they read the contents of the window, and for that reason there's a good chance that - initially at least - they'll approach it with an open mind. The mental process is more likely to be, "Hmmm ... what's this page?" than, "Here comes another ad." In other words, they're less on their guard, plus they actually see your message in full before deciding whether or not to close the window.
Even if the visitor does notice the page open and (if a Windows user) appear on their Task Bar, with pop-unders being less "in your face" than a regular pop-up ad, the potential to annoy is lessened, again reducing the chance of the window being closed without viewing its contents.
A Clever Use of Pop-unders
Using pop-unders to subtly open an entire second site behind the front page was pioneered by SiteSell several years before it caught on in the rest of the marketing world (my first pop-under was actually a simplified version of their code). Whenever you visit one of their individual product sites, the main SiteSell site opens behind it, with links to the entire product range. Click on the link above to see it in action.
It works like this:
Say you go to the product site "Make Your Words Sell!" but don't purchase immediately. Since the site has a strong back-up response, you probably sign-up for the free ezine. But even if you don't, SiteSell haven't lost you yet ...
You carry on surfing, and eventually close the front window. As you do so, the main SiteSell site appears, and you see a familiar name. If you're anything like most people, that alone incites some curiosity. You scan the page and take a look at the full list of SiteSell products. Having recently looked at one of those products in detail you're likely to be in a fairly receptive mood....
SiteSell get another bite at the apple - possibly several more - in one sitting. And doing it with a pop-under means it doesn't interfere with the sales process at the individual product sites.
Few webmasters use pop-unders in this way though. More often than not the the pop-under windows on their site being from one traffic exchange program or another.
Some high-traffic Web sites regularly sell pop-under ad exposures in much the same way as banner ad space. That's fair enough, and can be very worthwhile if appropriately targeted. Watch out though for sites that offer to send X amount of visitors to your site for a certain fee, if it turns out pop-unders are used to generate the traffic.
In most cases your results will be a disastrous waste of money if you set your homepage to appear in the pop-under window. With pop-under traffic, 99% of the time the attention span just isn't there for that. In most cases people will look at your page, but their curiosity will be short lived if it doesn't contain something that jumps right out and pulls them in. The SiteSell example I mentioned earlier is vastly different because the surfer has already pre-qualified themselves as interested in SiteSell products, having already visited one of their sites.
To make buying pop-under traffic viable, you need to create a special offer page for the purpose. You'll get best results if this is a short page with a powerful headline selling an attractive free offer designed to collect email addresses for further follow-up.
Use pop-unders on your site as you would regular entrance pop-ups. Just remember that sometimes "in your face" is exactly what you want!