Allow a gentle rounding of the back at address
The coach says a gentle, natural rounding of the back at address is fine because the back is designed to bend that way — not a hunchback extreme. He warns against the opposite of being told to push your butt out into an S posture with a stiff, straight-feeling lower back, which tenses the right leg and makes it hard to take the club around. In his experience that S posture puts stress on the lower back, hips, and knees and creates injuries.
Channel: SagutoGolf
Watch official source at 3:24