I don't have any inside information, but as I understand it's basically just a spending commitment. The online auctions still take place, however the purchaser will gain some sort of bonus for agreeing to a certain amount of spending over the next year.
The deals seem to be rather secretive, but from what I can tell bonuses can be anything from a straightforward discount, to a guaranteed amount of impressions for some campaign, to things like guaranteeing that the client will win a certain percentage of auctions above some price.
The deals seem to be rather secretive, but from what I can tell bonuses can be anything from a straightforward discount, to a guaranteed amount of impressions for some campaign, to things like guaranteeing that the client will win a certain percentage of auctions above some price.