I'm not sure how much clarification we're going to get, given:

* Northwest reserves the right to limit this offer in certain circumstances where it is infeasible, such as smaller aircraft service within Asia and/or during major world events, such as but not limited to: the Olympics, major auto shows, Super Bowl, etc.

This is a pretty expensive benefit. Assuming that it leads to an additional bump 50% of the time, and the average true cost of a bump is $200, this costs $100 a pop. Not surprising that they try to weasel out of it. My guess is they only do it when a flight is oversold by < 3 or something.