Not picking on you whatsoever, but I do bristle when I hear "shoulders back." (And God knows, I heard it a lot over my years riding/competing.) I like "pelvis back" much better, but no one really gets that and for obvious reasons begin to gyrate like Elvis, so I go with "hips toward cantle" or "hips away from thighs." Point being, securing yourself in the tack is the result of adjusting your pelvis, not your shoulders.
When blood is the beverage of choice, the sharpest fangs feed first.