When you set up to the ball your trail wrist is at a 90 degree angle to your swing arch. As you swing back to waist high your trail wrist is still at a 90 degree angel to the swing arch. As your arm swings to the top of your swing your trail arm externally rotates in your shoulder socket. That rotation makes the club face look as thought it has opened up. Then as you swing down the arm internally rotates back so that at waist high the wrist is back to a 90 degree angle to the arch. It then stays in that same relationship until waist high on the follow through. At that point your lead arm continues to externally rotate.