Why?

Why does my back hurt? Very likely I have been leaning backwards, my lower back tensed up and carried my body weight instead of the legs. My mind should be the boss and tell my body to stand upright. When I relax and flex my hips, I would feel strength in the legs.

Why do they call it abdominal breathing? The chief breathing muscle is the diaphragm and abdominal muscles support it. So the belly will bulge forward when we breathe deeply.

The lattisimus dorsi(back muscle or lats) supports breathing out, it is also called the "cough muscle". The back hurts when you have a dry persisting cough.

Sometimes we can feel our teacher's back to get an idea of good breathing.
When we use all these muscles correctly and our posture is right, we will naturally use our legs to carry our weight