At impact, your clubface points to the left of both your target and your swing path.
Imagine you're making a karate chop at a board with the side, or heel, of your left hand on your downswing. This is the swing thought that helped Billy Casper get his hook under control. By leading with the side of your left hand, you delay the rotation of your forearms just enough to prevent the toe of the club from passing the heel too early. Thus, you can't turn the face over. Think of it this way: The back of your left hand mirrors the position of your clubface at impact, so if you can keep your knuckles pointing right of the target, your clubface will look that way, too. If the face looks right, there's no way you can curl the ball to the left.