Let me explain. The trade promotion authority just means Congress will vote up or down on the deal: if they like it, they'll buy it, and they won't amend it. If they don't like it, they'll can it. That's the way it is.
They're shaping it through the process, so they can buy it. If the trade promotion authority expires, as it has in the past, all they do is look at it again and use it to try to leverage more concessions. If it looks as if they're going to get a deal sometime about the time it's going to expire, they'll find a way to extend it, but they'll try to extract more concessions, or put more conditions on it.
It's just a vehicle they use, because the President can't deliver without them. We found in the Tokyo Round in particular they would agree to things; for example, they agreed to something on a customs valuation in the Kennedy Round and Congress didn't deliver on it, so the next round they went into this trade promotion authority thing that said Congress wouldn't try to amend the deal once it was put to them. So that's what it's all about.