Whenever the intention is to work the spine, with lateral flexion, rotation, etc. then it’s important to keep the lower body in place so it’s not creating a fake range, i..e. making you think that you are creating a range in the spine but are actually moving your hips. Another important reason to do so when you are working with rotation is that if you don’t pivot on the foot when turning your hips, it will create a force on the knee joint that can be damaging.
When performing lateral or rotational movement, the best thing to do is to push the opposite leg into the ground, meaning, if you are rotating to your left side, then you press the right leg into the ground to resist it coming along or creating a fake range.