Your diaphragm, tucked up inside the base of your ribcage, is shaped like Darth Vader’s helmet or like a jellyfish. Its primary action is to lift and open your ribs, making space for breath to happen. It's part of the magic link for better posture and less pain, so it's really helpful to know more about how it works.
To explain, I will further belabour the jellyfish analogy because jellies pulse in the vertical plane (up/down) much the way diaphragms do:
I made the infographic above using screenshots from this video, in which you can watch jellyfish is live action (pretend they are diaphragms without ribcages, okay?)