There is a detailed post about this on the Guts Tummies Bellies Support Group – about the vagus nerve.
It gives information about how having a hiatal hernia (part of the stomach poking up through the diaphragm instead of staying below it) leads to all sorts of problems including pressing on the vagus nerve (a key nerve for heart liver lungsstomach…)
Not surprisingly it also involves a lot of discomfort and difficulty with breathing smoothly and so ‘clutching/tightening up across the pain area’ is one response.
A very simple exercise is being tried to reposition the stomach below the diaphragm and reset the vagus nerve. I’ve been trying it with a powerful good effect on my own breathing posture stamina and I suspect gradually weight.
In regards to posture if I’m bent over and need to straighten up an internal mental instruction to “push my stomach down – from inside” has the effect of not only pushing my stomach firmly down and giving me a strong front ‘girdle’ but it also enables me to straighten up without doing anything to my back at all.