Shoulder Check
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.