Sway back adjustment

I am trying the bistro dress shift dress modification (the sidewalk cafe dress?). I’m doing a muslin and am pretty happy with the fit in the front, but struggling with fabric bunching in the back. The blog post mentions a sway back adjustment and I trying but not gerting it right. Can anyone point me to more information or a good tutorial? I am new to sewing dor myself and this is my first Liesl and Co pattern and muslin (although I’ve sewn a lot of O&S for my daughters).