Compare the second and third frames to see the sequencing of Ernie's club, hands, arms and chest and how they coordinate. Notice how his chest hasn't turned until frame No. 3, after the club and his arms have moved significantly.
One of the keys to Ernie's power is his wrist action. Notice the complete wrist set at the midpoint of the backswing, in frame No. 3 on the left. Ernie's left arm is parallel to the ground, and he has set the club at more than 90 degrees in relation to the ground. With this sort of leverage—and the club swinging in balance—Ernie can simply turn to the top and then let his swing uncoil smoothly.
Ernie's shoulders are in great position at address: He pulls them back so his chest feels "wide," and his right shoulder is slightly below his left.
Like some of the great swingers who came before him -- Sam Snead is the best example -- Ernie begins to move his lower body back toward the target just before the club gets to the top.
Ernie uses his right side so well in the downswing. His right shoulder is moving down and slightly under his left. Many poor players swing the right shoulder out toward the target line, or "over the top."
You've heard the phrase "hit against a firm left side" -- this is a perfect picture of it. Ernie's arms and chest are uncoiling around his left leg, whipping the club through with great speed.
One of the challenges Ernie faced after surgery on his left knee was to avoid favoring it -- consciously or unconsciously. He sometimes struggled in his ball-striking with the long clubs because he would come up out of his posture before impact to try to alleviate pressure on the leg.
In these photographs, you can see how Ernie retains the angle between his upper body and lower body from address through to the finish. He's all the way back to where he was before the injury.