The armchair theory's nice - it goes something like this: the more you flex the rod, the greater the tip speed when it straightens - when you perform a haul you force a deeper flex into the rod and when that one straightens it does so like a bullet. Which of course explains why the Catapult Cast is a superior distance cast.
The flyline is travelling at the speed of the rod tip plus the speed of the haul. We know that. Newton told us. And he knew what he was talking about. Mostly.
Okay, I agree, the haul can flex the rod some more, but I disagree that this is what we are doing. Figure this one out - let's say that we were to haul during the stroke and finish the haul at the precise point that the rod begins to unflex, what then? Now we're in the prize position of having the flyline travelling quicker than the rod tip and this before the rod tip has straightened. Do you really want this? I think not.
Try this - go out try the double haul releasing line immediately upon completion of the haul. If you release the line before the rod has unloaded the cast collapses right? I bet that you'll find that this doesn't happen and that you too are hauling through the stop.
erm, this isn't the b-all and end-all on the matter and I'm having a discussion with some extremely good casters who completely disagree with me - but I figured WTF, let's all talk about it :-)