The part played by the left arm—the controlling factor in the good golf swing—starts at address. It should be neither stiff, rigid, nor locked in joint at the elbow but it should be comfortably extended. Throughout the backswing, the left arm should remain firm and extended, for this enables a golfer to maintain the wide arc that builds up power and, moreover, promotes accuracy by helping to groove the same arc on swing after swing.
As many of us see it, the start of the downswing is probably the most crucial part of the entire swing. At this point many golfers make the costly error of letting the left arm virtually collapse. When this happens, the right hand, arm and side immediately take charge. Once in their grip, the player has no other alternative than to hit "from the top of his swing." He moves the club outside the correct line, and consequently he has to pull it back across the line of flight as he contacts the ball. The result of this hitting from the top is a slice if the club face is open, a pull or a pulled hook if the club face is shut. How do you avoid this? Make certain your left arm remains the controlling factor as you start down from the top of the backswing.
from HAROLD SARGENT, East Lake Country Club, Atlanta