Now what is early extension. It is when on the downswing, your hips move toward the ball instead of staying back and rotating. When your hips move toward the ball, the head tends to pop up which causes a multitude of errant shots. These include 'topping', 'flipping' the hands and the worst one, contact with the heel or 'shanking'.
Early extension is actually very common in most players' swings. Even at the top level there are some players that do early extend but it doesn't cause too much grief for their game. It all depends on the degree of early extending.
The main cause is that when combining hip turn when hitting through the ball, the posture tends to move out of position. This includes pushing the hips forward, without rotating effectively. I will often say that the biggest difference between a tour player and an amateur is their ability maintain their posture while they rotate through the ball. It is the most athletic move in golf.
When the player is trying to transition into an elite player, you will often see them working hard possibly at the gym to strengthen their abdominal muscles and lower back so they can maintain their posture while rotating through impact.
If you watch any top player, you will see their glutes push backwards at impact while they rotate through the ball. I always say that when you have achieved the perfect position at impact with your body, a person standing behind you should be able to see your left glute at impact as well as your hips staying back. If you can do this it means your rotating your body effectively while maintaining posture.
If you find that you are early extending, there is a great drill that you can use. You can put a chair behind you, gently resting up against your glutes. On the backswing, the right glute should remain on the chair and at impact, the left glute will be gently touching the chair. After impact through you can come off the chair.
If you do this you will maintain posture and rotate effectively through the ball.