when we become stressed, we activate our sympathetic nervous system to get our body ready for action; this includes breathing faster. When we purposely slow down our breathing, it in turn triggers the parasympathetic nervous system to calm us down, and as that system kicks in, the other physiological changes brought on by stress are reduced. Deep breathing is universally recognized as an excellent calming tool, and is a major part of meditation practices. You have heard of it before because it works.
