Your spine actually gathers when you inhale and lengthens when you exhale.
This might not be mind-blowing to you, but it's kind of revolutionary when you think about how many mat-based exercises ask you to elongate as you inhale, when your body is doing exactly the opposite. I only learned this little fact last year in reading about body mapping and singing, and it rocked me. Unsurprisingly, an accurate understanding of the mechanisms of breathing is fundamental to healthy, free singing, and most of us poor voice teachers perpetuate a lot of well-meaning misinformation about how breathing works. For example, when we tell people to feel taller on the inhale, it usually results in muscular tension somewhere--I would guess in the mid-back for most people.
Here's a helpful blurb about what your spine's doing when you breathe:
One of the most profound sources of buoyancy in our bodies is the gathering and lengthening of the spine that happens during breathing. The gathering occurs all along the spine as we inhale. It is partially due to rib movement. As the ribs move closer together when we inhale, they bring the thoracic vertebrae closer together. Another source of the gathering is the general deepening of the curves of the spine as the body deepens on inhalation.If you bring your awareness to this gathering and lengthening and allow--rather than force--it to happen, the net effect is one of freedom of motion and elongation, which it would seem is what the yogis are after anyway. And the sense of release on the exhale that allows you to twist more deeply into whatever posture you're working owes much to the natural lengthening of your spine.
When the vertebrae gather together, the springy discs of cartilage between them are slightly compressed. As soon as the work of inhalation is released, the discs of cartilage spring back to their full height. In other words, the spine lengthens as we exhale.
What Every Singer Needs to Know About the Body (Malde, Allen, Zeller)
I'm just saying.