As you can see in image 1 above, I have maintained my posture while rotating my shoulders and upper body to get the club to the top of my backswing. I have lots of energy and power stored, ready to be unleashed in my downswing. My head remains in the same position as it was at address, because my spine angle has not changed.This is vital if I want solid contact with the golf ball.
If I lose my spine angle during the backswing (as per image 2), I gain height, my shoulders rotate on a flatter plane, and less potential power is stored as my core muscles are not really working. At some point during the downswing I will need to lose some height if I am to make good contact with the ball. This can only lead to trouble!