On the other hand, breathing through your stomach (also known as diaphragmatic breathing) allows for more oxygen intake at a lower breath rate. (wellnessnova.com)
This demand for additional oxygen under exercise stress is met initially by increasing tidal volume (TV) as evidenced by the initial changes in ventilation we witness at the onset of moderate - intensity exercise (increases in TV precede increases in breath rate). (blog.nasm.org)
The noticeable result of which is better endurance, a lower heart rate at an intensity level that previously would've had a higher heart rate, as well as slower breath rate for the same perceived level of exertion. (drrosedale.tenderapp.com)