Compare 3 videos of Stefania Boffa. Picture 1 is the still of the video one frame before contact where you see Stefi reaching with her arms without stepping forward positively, which causes her upper body to collapse and her behind to stick out (typical picture of negative footwork)

Still of video 2 one frame before contact shows slight overreach but very good forward step so the upper body does not collapse.

Still of this back view video is perfection with good upper body posture leaning against the ball and beautiful forward step leading to a perfect return.

Lesson: If the legs do not step out positively on the stretch the upper body will collapse. Coaches can spot this lapse easily because the behind sticks out. It becomes difficult to hit up the back of the ball, or to follow through fully with power and control badly compromised.