remnant/surplus ad space - what gives?

Whats the deal with online remnant advertising and last-minute ad discounts? Will it ever be made freely available to buyers online in the future? I've been personally invetigating the issue but the only info Ive been able to find thus far about online remnant advertising is a blog - and some articles here and there. Are there are any sites out there that offer this? What exactly happens to this remnant inventory? Any ideas?? I'm interested in learning about remnant inventory and potential last-minute discount sales across all media, radio, TV, print, you name it!

Get a bunch of advertisers to bid on spots. When the high bidders budget runs out you move to the next high bidders, etc.

Or just use an ad network that will do it for you.