I think it is when you can trade in your current eVGA card like a 6600GT and get a more expensive card. So lets say you wanted the 6800GT. You trade the 6600GT in and pay the difference.
I think ATI still has it's trading program. If you buy one of their cards and send in your old one with proof of purchase (doesn't matter what kind of card, even Nvidia) then they send you back $50 USD or $80 CND. Now, this is only with the ATI brand of ATI chip cards. The Powercolor, Sapphire, Diamond (etc.) brands of the ATI chips won't be valid.