The default ad code is a setting in Popads most people have no use for – or no idea how to use it in the first place. However it has proven to be quite profitable for me to drive traffic to my webcomics. The way it works could be explained better on the page, but it’s not rocket science either. Whenever no ad can be displayed either because of a lack of advertisements or because your minimal bid was too high the default is shown in the popunder instead. Simply put the url of your website here (tip: If you have a webcomic use the link to the random comic page to spice things up) and you are good to go.
The views should not be underestimated, both the bouncers that close the popunder without looking and actual viewers. By running analytics I can see that I get a fair amount of actual viewers this way and it costs me nothing to acquire them.