The diaphragm needs to be thought of in relationship to the spine and the area underneath the diaphragm.
Try this trivial exercise to understand extremities of this. Crunch yourself up in a ball, and try to breathe using the diaphragm, and notice its difficulties. Then go into a military stance posture and breathe noticing its effects on the diaphragm.
When one crunches up, it is extremely difficult to get the diaphragm to exert pressure. In military stance, notice that the area immediately underneath the ribs and its lower back is strongly supported by the viscera and muscles underneath, and by the upward thrust of the lower back. But military stance has greater diaphragm movement than the crunched up position, even though, because the ribs are taut, breathing is harder.
Now, slightly back away from the military stance, while keeping the “support” from the lower back and viscera to keep the upper chest (and hence also the diaphragm) uplifted. You should now have a freer to move diaphragm, in a simplistic explanation. (The complicated explanation further involves the pelvis, neck-throat, upper chest, gravity, emotions, and head.)
Most of the postings here involve how one breathes, the vocal cords, the sounds. I suggest, instead, to focus on the primary anatomical parts (head-leg, pelvis, ribs, etc. and not the ones with Latin names) and determine freedom of diaphragm based on these.