Notice the weight is already set on the left sided, the left hip being the focal point. The left knee is slightly bent to make room for the extra weight on the left side at address, and the right leg is straighter (because it has to stay attached to your body) but slightly relaxed. Notice the arms are straight but relaxed and the grip of the club is centered with the middle of the body. From there, with most of the weight still on the target side leg and centered on the left hip, the shoulders initiate the backswing move s the club rises, seemingly automatically to the finish of the backswing (using a 7 iron). Once the body reaches the finish of the backswing the torso begins to uncoil in unison with the arms and hands. Everything moves together (staying connected) dropping the club face into the impact area, delivering the club on plane, and releasing the face and golf ball straight at the target. The left side (target side here) stays straight as the weight moves through the shot,posting onto the left leg. The head remains steady and basically in the same place through the swing, allowing a full, balanced follow through
From the back view or the second swing in this sequence, notice the power and club head speed that is generated by releasing the club with little effort down and through impact. Also notice the turning or rotation (not moving or swaying) of the left hip, and the “posting up” (in modern terms, of the left side, like hitting against the left side as the body rotates through impact and again into a relaxed and balanced follow through position.