If you're not familiar with the term stacked, as it applies to the golf swing, you should be. Basically, in a sound swing, the upper body should stay relatively balanced over the lower body without any significant leaning in any direction. At the far left, notice how my chest, shoulders and head are directly over my right thigh. I haven't swayed in the backswing, but have simply coiled my chest over a flexed right leg. This is critical for loading up and creating power. At the upper right, you'll see that my backside is sticking out rather than collapsing closer to the ball. This shows I've maintained my spine angle, which is also critical to a powerful swing.
"Allowing the majority of your body weight to move to your front foot at the top of the backswing is a huge mistake."