What Your Teacher Doesn’t Want You To Do
Now that you know the basics of the underlying anatomy, let’s look at how it applies to asana. In most poses we are either trying to find and then keep the natural curvature of the spine or in the case of backbends to evenly arch the spine toward the front body. Because upper backs are weak and lower backs bend easily, many people have a hard enough time bringing their spines to neutral when they're simply standing in Tadasana (Mountain Pose). Add the effort involved in more complex asana and that task becomes more and more difficult, so they revert back to their natural tendencies. For a lot of people, that means the lower back area overarches, the pelvis tips forward past neutral, and the abdomen and lower ribs puff forward.
This happens because teachers often say “Arch your upper back, lift your sternum, widen your collarbones,” etc, which correctly instructs people to get rid of the extra hunching and rounding in their upper backs. But because that's really hard work, students often arch the easier part, their lower backs, and tip their pelvis forward, which makes their lower ribs jut out.