Occasionally on tour you'll see a player, including me, cold top a driver or a long iron. In fact, it's the most common way really skilled golfers flat miss a shot. The cause is almost always trying to kill it, which generates an acceleration of the legs and hips that in turn over-delays the release of the clubhead into the ball by the hands and wrists. As a result, the swing arc does not reach full length at impact.
Another cause is swaying. Lateral motion in the backswing moves the swing arc backward. If it isn't then moved forward in the downswing, the low point of the arc is behind the ball, often causing a thin or topped shot.
Topping the driver is not as much of a problem today. With modern deep-face drivers, you can tee the ball so much higher. But it still happens with irons and fairway woods when the ball is on the ground. The problem is still the same—the swing bottoms out before the ball so the clubhead makes contact on the upswing—but I'd look at the cause a bit differently today.
I've learned that topping comes from getting out of sequence. A golfer who tops the ball usually releases the club early on the downswing instead of starting down from the ground up. You get a cupping action in the left wrist at impact, with the clubhead moving ahead of the hands.
You need to stay in sequence going back and through. If you swing everything back together and keep your head steady, you'll be in position to start down with your lower body. Then you can deliver the club on the right arc into the ball.
JACK NICKLAUS writes exclusively for Golf Digest. In this series he looks back at his classic lessons published in the magazine.