Premise: Through impact, we want the weight in the left heel to counter-act the outward pull on the club.
Your weight will move to where it isn't. If your weight is on your heels in the setup, it will move to the front on your feet as you go back to maintain balance (pulling your bum off the "tush line").
Your sensory system does not care if you lose you spine angle, it only cares about keeping you from falling over.
The Fix: Make sure you are closer to the balls of your feet as you setup to the ball. To steal a tip from Redgoat, feel like you are standing up on the pedals of a bicycle. Then, your weight can move to the right heel as you complete the backswing. Also, try to keep the clubhead outside of your hands as long as you can (this is a FEEL) as you take the club back.