Here's his blog entry on the subject:
You must know this about the deadlift
The visible range of motion change from pulling with a tight lower back and abs but relaxed upper back vs. a tight upper back is obvious in the video. I've always been taught scapula tight at the start, but I'm going to try to pull this way next time and see how it goes.
Don't mistake his advice for allowing a lax lower back. A loose lower back won't get you heavier weights, but it sure can get you hurt. This is a question of upper back tightness only.