To be truly correct and effective, the start of the downswing must be initiated with the lower body. During the backswing the weight of my body has been applied to my right side (about 75%). This weight then has to be shifted back to, and then through, the ball. Your hands and arms do not do this for you. Your hips and legs bring you back to the ball, with authority.

You see other athletes using this same lower body / mid-section rotational technique in other sports as well–tennis players hitting ground strokes, baseball players batting, shot putters tossing 16 pound balls of iron, and boxers throwing hooks and crosses. That's because it works.

Notice that the 90° club / arm angle has not lessened at this point.

Obviuosly, the bulk of your exercise program must be geared toward the legs and trunk, which increase your swing power when uncoiling.