I have avoided this thread (to the relief of all, I'm sure...but now I have you....MOOOHAHAHAHAHAHAAAAA) because this wrist action has absorbed an inordinate amount of my time for the past year, and I can’t possibly write a short answer. Sorry boys. But the good news is that I’ve sussed it out on my end….for ME. Maybe not you. After trying a cackajillion different approaches to deal with pronation of the left wrist, I have come to believe something really scary: it is like worrying whether you breathe in or out on the backswing.
First, loop. When I took the game back up like 5 years ago, I had a problem with over-cocking my wrists, that was identified by my one and only foray to a golf school (Ben Sutton, Tampa area, pretty good really). On my intro video, I looked like Daly, with the clubhead almost hitting my shoes at the top of the backswing. At that time I hit the ball pretty well, really, very straight, but not too far. And I couldn’t score for sh!t. The pro I had let me swing my way for the first day and a half for the 3-day school, and then he couldn’t take it anymore, and completely deconstructed my swing. I mean completely. (The man utterly effed up my game for more than a year. Problem is, all the stuff he gave me was right, and has since made it into my swing) One of the first things he “fixed” was my backswing. For me, he told me to only try for a ¾ backswing, and to limit my wrist ****, to gain consistency and power. Well, to make a long story short, I did this, and although I felt like I was hardly cocking my wrists at all, I was perfectly parallel to the ground at the top. So like Bravo et al have said, you are cocking your wrists, though it doesn’t feel like it. And perhaps, for you, it shouldn’t.
Now, Dave. This supination thing. Welcome to my hell. 
As I’ve written to you before, Hogan's 5 Lessons is one of 3 golf books that I read all the time. With regard to supination, he says specifically that it is a key to being a good player, that all good players supinate at impact. He then gives the example of Jimmy Demaret, saying he really “slaps at the ball with the back of his left hand” (or something close, the book’s not in front of me). If you go to the golf channel site or anywhere else that they have the pro swings on video, look at their left arms (or lead arms) at impact. They are supinating. Look at Els, Vijay, Charles Howell, hell, look at Corey Pavin. Supination city. I know Hogan doesn’t do all the things in his swing that he talks about—that picture of the arms wound with rope at address leaps to mind—but on this one, I think he’s pretty literally accurate. Their wrists all flatten out AFTER impact, maybe 12-15” past the ball, when they all have that nice extended triangle happening.
With regard to flat, flat is ok at impact. Pronated is not. So if you’re flat, and comfortable hitting the ball that way, and you like what the ball does, be flat. And be happy about it, dammit, ‘cause that’s your swing.
For me, I’ve spent a year trying to force that supination. And that’s where, I’m afraid, it wasn’t happenin’ for me. Anything I tried to do mechanically, from bowing my left wrist at the top, to hitting at it with the back of my left hand, to maintaining my frame through impact, to a half dozen other attempts, none worked repeatedly. My hands fought each other, or my downswing slowed, or I fatted shots, damn, every type of crap.
What eventually worked, for me, was envisioning the head of my irons cutting an angle through the ball and into the turf. That simple swing thought (right after "seventeen" for tempo) gave me supination, because you can’t cut through the ball like that if your pronating, or even flat. I also start my backswing now with a forward press, which starts my hands out ahead of the ball. So if I return them to the address position, they arrive ahead of the ball. Every action produces an equal and opposite reaction. (With apologies to Issac Newton. Or was it Einstein? I think it was Newton).
So the only way I was able to repeatedly supinate at impact, was by not trying to do so. God, I hate this game.
Hope this helps. I feel like I’ve just given birth.
