Deep breathing makes you relax
Diaphragmatic breathing, also known as deep breathing or belly breathing, send signals to your body by telling it to relax. When you are in a stressful situation, your sympathetic nervous system is stimulated, which makes your breathing fast and shallow.
However, by adopting a slower and more controlled breathing, you trigger the vagus nerve, which is the main component of the parasympathetic nervous system. Triggering this helps you calm down and relax, so you can feel better and think rationally again.