So to sum it all up: structural integrity should by all means be kept. We do try to engage the forward hip when striking and thrusting forward, but overemphasis of this position causes the aforementioned affliction - quack quack...
I'd just like to tally that with this:
Osensei in Iwama. Someone please correct me if I'm wrong, but the defining point of the DDP is something that is not always easy to see. I think the DDP has an actual kink, or exaggerated curve in the lower back, turning the hips down and the sacrum up. Osensei isn't doing that here and Ethan's explanation prohibits it.