The golf swing in its most simple form is a circle. The radius of this circle, back and through to the finish, is defined by your left arm position (for a right-handed golfer). Obviously, it needs to be wide—the wider the circle, the better.
When a wide arc is established during the backswing, a golfer becomes more able to drop the arms in the correct position on the downswing, which allows the left arm to release through impact and fold correctly in the followthrough. The left-arm fold on the followthrough is key, as it keeps the club on the correct plane and the ball on the target line.
A good ball striker creates a wide, powerful arc on the backswing by allowing the wrists to hinge naturally, which places the club on the proper plane. More importantly, they use their flexibility and upper body turn to move the club to the top without the left arm having to fold. High-handicappers tend to do the opposite. First, because of what's likely a faulty grip, they fail to properly hinge the wrists, impeding their ability to get the club on the correct plane. Furthermore, since these golfers don't turn as much as they should, they move the club to the top by folding the left elbow. In this position, the high-handicapper loses the swing radius established at setup and, typically, ends up coming over the top, a move that exacerbates the problem by limiting correct rotation and release of the left arm post-impact.
You don't need to swing exactly like a Tour player to strike the ball well, but don't do the exact opposite. Keep the left arm extended on the backswing and let it fold on the follow-through. If you've been struggling with a left-to-right ballflight or fat and thin shots, focus on your left arm.
Correct use of the left arm begins with the grip. Check to see that the handle lies diagonally across the fingers of your left hand, as opposed to the palm. When the grip lies correctly in your left hand, hinging the club on the backswing becomes natural. If your wrists can hinge on their own, you'll be tricked into hinging your elbows.
Correct use of the left arm is also dictated by your shoulder turn. Good ballstrikers synchronize their arm swing with their shoulder turn. This synchronization facilitates both a solid turn and a straight left arm. High-handicappers usually stop turning their shoulders too early, but continue to let their arms swing back. The result is a shortened swing arc and a collapsed left arm.
Here's a drill that will help you keep the left arm extended on the backswing and fold it correctly on the followthrough. First, make sure that you're holding the club correctly in the left hand. Second, take your right hand and place it on your left wrist. Your right thumb should point to the ground and your palm should face the target.
Swing back and keep your left arm extended by pulling it across your chest with your right hand. Keep your hands away from your head. Focus on maintaining the radius you created at address all the way to the top of the swing.
Now, swing down. As you begin to release through impact, physically rotate your left forearm with your right hand. This is how you square the club at impact. Once you begin to rotate the left forearm, you'll feel how the left arm correctly folds on the follow-through. Do this several times until it feels natural. When you take this new feeling to the course, you should experience a more efficient swing and longer, straighter shots.