We want to maintain a neutral spine posture during the squat - this means ribs stacked on top of the pelvis. Over extending or letting your back round at the bottom move the spine out of that natural position which can long term create problems including injury! . The solution: focus on creating that stacked position at the start of you lift, for some with mobility restrictions that lose this stacked position at the bottom a heel lift can be a helpful tool to maintain more optimal positions. . S