In a yoga class, you may hear your yoga instructor say something like "Breathe into your heart" or "Breathe into your abdomen" or even "Breathe into your hips" or another part of the body seemingly unrelated to breathing.
As far as hips, this kind of statement has some validity.
Your breathing muscle is called the Diaphragm.
The diaphragm flattens when you inhale. Try it!
Can you feel your abdominal contents being pushed down by the incoming breath? The diaphragm is a dome-like muscle that flattens as it contracts on the inhale.
As you exhale, the diaphragm relaxes and goes back to its dome-like shape.
The diaphragm attaches to the ribs and the lower back (lumbar spine vertebrae). Take a look at the above image.
Guess what else attaches to the lower back vertebrae? ... The Psoas. A very deep hip flexor - you can see the Psoas fibers on the image cut right along the diaphragm attachments.
The fibers of the diaphram and the psoas criss cross and intermesh. A really deep breath gently pulls on the attachments of the Psoas and stimulates them to soften and relax.
This is a really great thing!
The psoas is tight in so many people and you can help release it yourself just by breathing deeply. Definitely try it at home ...and at work.