A tight diaphragm can feel like a belt around your middle or a knot in the middle to lower back. Yoga brings muscles into focus especially the ones that are tight and tied up. Tension in our principle breathing muscle compresses, tugs, and twists us out of shape and out of round. It is a tension that has proved useful in shaping our spine, controlling our emotions, and pressurizing our brain, a foundational tension to our physical and mental conditioning. There comes that time for lengthening, loosening, and releasing the diaphragm, freeing up the breath. And that is called yoga, pranayama, Qi Gong (Chi Kung in the older spelling).
The diaphragm viewed from below. Notice how the crura, the tendons that attach it to the spine also wrap around the aorta the main blood vessel in the body and have it in its grip.