When you inhale, your diaphragm moves down. Your heart gets a little smaller. Blood flows more slowly through that larger volume. Exhales slow the heart down. Inhales speed the heart up. If you increase the duration or the intensity of exhales relative to inhales, you'll calm down. People who have apnea don't do this well. Apnea is very dangerous. It actually will shorten your life,. lead to all sorts of issues, not good. You can also use physiological size between rounds and jujitsu to calm down if you want to calm down...you're using them all the time anyway. We're just not normally aware.