Answer: When it is a SubSite.

I was just reading The Guardian and a new browser window opened behind my front viewer. Concerned, thinking that my computer had been infected with Gator technology, I decided to investigate. It turns out that The Guardian has just started allowing SubSites to deliver advertising across their publication.

I was a touch surprised by this, believing that The Guardian was above serving pop unders. You can therefore imagine my reassurance to find out that:

Upon opening a targeted page on the host website the SubSite is generated below. A SubSite is a web site in a full screen browser and not a Superstitial or pop-under advert, which only have limited interactivity and just a frame.

They go on to explain that Subsites are "not intrusive like pop-ups that cover the content of your site and annoy your visitors." As you can see from this example SubSites are definitely not pop unders, and are in no way annoying. Yeah right - still I suppose that they deserve some credit - The Guardian's management bought this drivel, although I doubt that their readership will be quite so easy to persuade.