A Malaska Golf Member comments on how the left arm immediately rotates on the downswing.

This is external rotation. Mike explains that the right arm is also rotating. As it goes back on the swing, it externally rotates to the top; then, as the arm comes down, it internally rotates into the ball.

When you take your left arm on the backswing, it does the opposite of the right arm. It internally rotates to the top and then externally rotates on the downswing and partially through the follow-through.

Mike demonstrates a swing holding the club with only his left arm and hand. He says his left arm is rotating in his left shoulder socket. He makes the same swing with his right arm.

Combine internal and external rotations of both arms, and you have your golf swing.

Mike shows how the right and left arms rotate in either shoulder socket. It is not your hands and arms rotating but your shoulder sockets. Your shoulder sockets must be relaxed. If not, they won't be able to rotate, and your hands and wrists start to twist, making it hard to time when you are trying to hit the ball.

Again, both arms' internal and external rotations match and are crucial to making the shoulder sockets work through the ball.