Here's a perfect example of trying to get some separation from the arms and the left shoulder on the down swing.
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The reason the left swing has a poor posture at setup, and has problems swing-wise is the grip. He's trying to hold the club with the clubface square. He needs shaft lean at address, which shuts the face in your hands around 25-30 degrees with a wedge, less with longer clubs. Crenshaw has shaft lean at address, therefore forcing his posture rightward to square the face.
Trying to hold the head still or move it backward in the downswing is generally not a constructive swing thought. Neither is trying to freeze tbe left shoulder or move it upwards. These conditions happen automatically if there is pressure into the ground through the left leg. In fact, the feeling can be one of using the left leg to push the body back away from the target while swinging the club down into the all. This automatically keeps the head back and the left shoulder in position.
Do you want a reverse K with iron? Why are these video all have their head behind the ball at impact?
Keep head behind ball, everything else follows.