CPM ads at the bottom of the page?

I see a lot of sites which show CPM style ads at the very bottom of the page. How are they doing this? Every CPM company I have looked into says you have to put their ads either above the fold or like 500 pixels from the top of the page or something. Do these sites have a special deal with the company? Or maybe they agreed to have their eCPM be considerably lower? I’d love to stick a banner down in my footer for a discount rate if that’s possible.

Original post by archard