What is this?

Hi Zach, The red circle surrounds the lumbar spine and the lower thoracic spine. The hollow-back look you're noticing is likely a strong recruitment from the hip flexors in transition, possibly combined with some lower back muscles to help the trail shoulder pull on the handle. I don't think it's a bad move, but it could lead to a later core activation and a late or no turtle shell. So I'd keep an eye on it. But I would probably attack it more in the gym at first, assess the core and see how the golfers strength and movement patterns show up. Best, Tyler