The development of a golf swing, as everybody knows, is a highly complicated procedure. Obviously precision is its essence, and a part of the precision necessary to make contact with the ball is the reasonably straight left arm and the pivot that causes the downbeat of the swing to move from the inside out. Very rightly this inside-out feeling of the swing is of great importance. However, in my observation an exaggerated inside-out swing is a hindrance to the follow-through because it tends to lock the hips, preventing them from completing their turn.
A simple and easy way to combat this common tendency is to address an imaginary ball one foot above the ground. Swing your club in a circle, keeping in mind that the club head must make the same "circular" swing beyond the ball as it did coming into the ball. To achieve this circular motion, your left arm, you will discover, must bend at an imaginary point after impact, just as your right arm bends on the backswing. The high practice swing tends to give you the feeling of inside-inside instead of the more cumbersome impression of inside-out. Actually, however, your club head is going inside-out until impact but then it continues inside the line of flight once the ball is on its way.
from HAROLD CALLAWAY Pinehurst CC, N.C.; Skytop Club, Skytop, Pa.