Speed and contact. That is all the ball cares about. The golf swing can be broken down into power accumulation, speed, and contact/delivery.
Power accumulation
is what the backswing is for. The golfer must make space for speed by coiling around the back leg.
Speed
The primary swing speed factory of the body is the arms and hands. The lower body, torso, and shoulders add speed but are primarily used for delivering that speed to the ball. A golfer creates the bulk of her swing speed by extending the arms and snapping the wrist.
This may be done passively or actively. I prefer to actively extend the arms as it creates more speed than the passive method. If you don’t believe me, just go outside and start throwing a ball sidearm again. Try the passive method — let the rotation extend the arms — and then try actively extending the arm and see which one goes farther.
Contact
Almost every swing fault can be traced to trying to use the arms to create contact instead of speed.
The purpose of the lower body, torso, and shoulders is to contact the ball, not primarily speed. While speed is created by all these muscles, the biggest mistake that golfers make is trying to get all the speed with the big muscles. Moving the body into a solid contact position is what you are after with the body.
Take a look at the picture of Jeff Flagg — World Long Drive champion — above. Pay particular attention to frames 2, 3, and 4. You may not be flexible enough to get anywhere near his position in frame one, but every golfer should be able to get the rest of these swing positions. Notice in frame 4 how when his lag is gone his right hand is in line with his right pectoral muscle.
Jeff has maxed out his club speed over his right side several inches from the static hand position of his setup. To compensate for the delivery of speed behind the starting position of the hands, the lower body, torso, and shoulders have moved the hands forward and down to crush the ball. Every good swing has this in common.
To get the feel of this speed and contact take a club and flip in so you are holding the clubhead side in your throwing hand. Start making some sidearm throws focusing on the arm speed and moving your body as little as possible. Then once you have this feel start adding body rotation. To make sure you are maxing out your impact put a club or alignment stick down in the middle of your stance. Focus on maxing your extension out past this line. Once you are consistently whipping the club past this impact line, start making a descending throw with your arms keeping your body motion the same. Now add your other hand and get both arms synced up.