[AlexCoventry]

It's unhealthy but anatomically possible to breathe while your abdomen is pulled in. https://www.medicalnewstoday.com/articles/319924

Try it. You end up breathing using your rib cage instead of your diaphragm.

[sah2ed]

That unhealthy form of breathing is called vertical breathing. The correct form is horizontal breathing [0] helpfully shared elsewhere in this thread.

0: https://www.artofmanliness.com/articles/how-to-breathe/