Here's an image to help you understand the correct shoulder turn. To keep your spine in line with the ball, as you should to promote solid contact, you have to rotate your shoulders in place. Think of your shoulders like the blades of a fan: They turn without the center moving (above, middle). If your shoulder center moves to the right as you swing back (above, right), you have to return it by impact, which is difficult to do with any consistency.
Drill to Try
Place two clubs in a line, with a 12-inch gap between them. Using a 5-iron, hit balls from directly between the shafts. If your shoulder and hip centers are in front of the ball at impact, you'll make divots on the target side of the shafts. If you're hitting the ground behind the shafts, your centers are stuck behind the ball.
Try These Two Master Moves
Let's break down the Stack & Tilt Swing into two basic moves: (1) a tilting to the left on the backswing and (2) a standing stretch on the through-swing. The correct sensation going back is a continuous tilting toward the target. As we said earlier, your forward tilt toward the ball at address moves to your right as you swing back. You have to tilt about 30 degrees left to get your spine vertical at the top. This tilting creates tremendous torque in the body.
The standing stretch that starts before impact and continues to the finish opens up the body to the target, which allows the hips to keep turning and maintain their speed. This springing up is a powerful lateral and upward action that initiates the forward flexing of the torso (below). Picture the energy that a standing long jumper creates when he launches himself into the air. That's the driving forward thrust from the lower body you should feel on the downswing.