Challenging exercise.. and wondering the same thing when I was doing this one. For me, seeing her left knee, I sorta made out where the pelvis could angle. And as for the ribcage, I think the neck opening is more towards us and more foreshortened than the answer provided. My sketch is may be also off. Correct me if I'm wrong, anybody.

So tense, looks like you’re trying too hard. Also, it’s hard to read whats goin on with the beans esp the ones at the bottom right.  Maybe try doing warm up of drawing circles and ovals til it flows naturally, to the size, shape and stroke pressure consistency you want. And remember to relax your grip more.  Then, instead of diving straight into drawing the bean from imagination, try to copy from stills of the video or an actual stuffed sock to better grasp whats happening at certain positions and angles.  Hope this helps. Happy drawing. 

Went back and redo, to emphasize the oblique more