With the exception of the Moe Norman-style swing (where the wrists remain in line with the arms at address and impact), the wrists are naturally hinged upward at address. As the club makes it back to parallel, they hinge a bit more. Harvey Penick, in one of his "little" books, stated that at parallel, the wrists have already hinged enough, and that the backswing can be completed with no additional hinging needed.
At address, the arms are naturally more together, with the elbows pointing down at each hip. But on the downswing, the left elbow is out, pointing more down the target line, and the right elbow is in by the side. Because of this, the hinging has to be lost just prior to impact. The left wrists should naturally bow to retain a bit of lag, keeping the club face square longer, and keeping the right hand from releasing too soon, but the wrists at and through impact are no longer hinged -- they're now in line with the arms. They have to be or they couldn't roll over -- well, they could, but the toe would be pointing up in the air.
That's why it appears at times the club head is resting on the heel, with the toe slightly up, at address -- it's the hinging at address causing that. However, through impact, the heel and toe (ideally) arrive in line to the ground together -- that can only be accomplished if the wrists are no longer hinged and are in line with the arms through impact. Pretty cool. I just learned this recently on one of Martin Hall's School of Golf shows.
Moe Norman purposely started his swing with no wrist hinge, since he simply wanted to start his hands in the same position they'd be through impact. Made since, and he sure made it work.
Nice thing about all of this is that with most all of the hinging occurring at address, you really don't have to think about hinging after that. Allowing that left wrist to stay bowed through impact though -- well, that takes some work. After 30 years of playing, I still don't have that one mastered. And since the bowed left wrist helps keep the right hand from rolling over too quickly, I guess that's yet another reason I fight the hooks.