One of the most central (and surprising to many!) movements of breathing is spinal excursion. The spine gathers as you inhale and it lengthens as you exhale, if you allow it.
Spinal excursion coordinates all the other movements of breathing. If spinal excursion is allowed to happen, the other parts of breathing occur as they should.
I like to think of the spine gathering in terms of the vertebrae getting slightly closer together. When this happens, the ribs can swing up and out in the front just that more efficiently. In other words, as you inhale, your spine gathers and your ribs swing up and out in front, blossoming like a butterfly opening its wings.