Hello, I am still learning this as well so my advice might be off, but I am assuming that the center line from the top follows the angle where the rib cage is tilting, which is forward. While she is bending forward and creating that 'fold' her pelvis is tilted outward so the center flows out. I hope I was able to help, and maybe someone else will jump in with a better explanation. Other than that, keep at it and it will make sense little by little.