Your wrists move in three specific ways during the downswing. Your wrist can hinge up or it can hinge down. Hinging up is radial deviation and hinging down his ulnar deviation. The second way my wrists work is called a flexion and extension or bow and cupped. The last movement of it is really part of my arm as well, which is pronation when the back of the hand is up towards the sky and supination is palm up towards the sky.

In a stock golf swing, if we get ourselves down to the last parallel position, let’s assume that my left wrist in this portion is flat. To get my wrists from there in my downswing to the golf ball I am unhinging or ulnar deviating and I am supinating or rotating my forearm.
.
Then I unhinge my wrist to get the club from up in the air to down to the ground. As I’m doing that I’m turning and I’m also adding some supination. At impact, my left wrist is completely unhinged down. And with that, by the time I get to impact, if I have a neutral grip, it has rotated all the way back to, towards the target.

Now let’s talk about from the top of your backswing. Let’s assume at the top of the backswing your left wrist is flat. Some players will have a flat left wrist and they maintain their flat left wrist. Some players have a flat left wrist and they go into extension. We would prefer you to go from flat left wrist and keep your left wrist flat, or to go from a flat left wrist into some form of a flexion.

So it’s adding flexion, adding supination and unhinging to get the club down to the ball. Those are the three main movements.

Flexion or bow and supination go along together. When you add some flexion, you get some supination for free. If you add extension or you cup your left wrist, you’re in big time trouble because that goes with pronation or against supination.

The point of this video is to explain there are three wrist movements. They’re happening simultaneously as you go. It’s not as though you’re making a downswing and saying, okay, I’m going to do this first and then I’m going to unhinge and then I’m going to supinate. They’re not broken apart. You’re doing them simultaneously. Just like if I walk, I’m doing a bunch of things simultaneously, like any other activity you would do.

The point of this is just information. So one more time, assuming I have a flat wrist at last parallel, the two main motions from here to impact are I’m adding supination or left arm rotation and I’m also unhinging. I’m unhinging with some supination to get the club down to the ball. Ideally I have a flat left wrist and I’m maintaining it. If not flat, I’d rather it be bowed or arched back from here. You can do that from the top. You can do that towards the bottom, or you can do that into the golf ball.

Source