It seems odd that we have to teach students to breathe. They seem to have done just fine prior to our expert instruction, and none have ever expired in my studio. Either way, this discussion can be brief. Go find a sleeping pet, or a slumbering child. What’s moving? Their stomach is moving gently in and out. For starters, do that. Don’t stop.
Of course, performance related breathing is a huge topic with different opinions and methods from thousands performers and teachers. Some unifying themes I emphasize are that for the diaphragm to move down, the stomach must move out. It’s good to expand the chest, but not if the diaphragm moves up into the chest cavity. I call this the “Saturday Morning Superhero” syndrome where Mighty Mouse puffs up his chest before proclaiming, “Here I come to save the day!” With that breathing method he’s not going to save anybody. Keep your center of gravity low; pretend you’re pushing a piano or dresser across the room. Practice your breathing during ensemble rehearsals, breathing in, filling from the bottom up with a mantra similar to this; stomach out, diaphragm down, fill and expand chest, pop a final little breath on the top of your chest, and in your neck. Now get everybody out of the way and fire your big gun.