Breathing and blinking are the only two actions of your autonomic nervous systems that you can bring under conscious control anytime you choose. As such, your breathing is the bridge between your somatic and autonomic nervous system.
Think of your autonomic nervous system as a big, shuddering, shaking machine that has only one control level sticking out from its side. Your breathing is that control lever, the one thing you can reach out and grab. When you control your breathing you control the whole autonomic nervous system. As discussed earlier, the autonomic nervous system has two branches: the sympathetic nervous system and the parasympathetic nervous system. Through proper breathing, you can control your sympathetic nervous system response, the fancy term for fear and anger. As mentioned earlier, uncontrollable fear and anger are the same thing, they are just two different manifestations of the puppy getting out of control. Tactical breathing is a leash on the puppy. The more you practice the breathing technique, the quicker the effects kick in, as a result of powerful classical and operant conditioning mechanisms.
- On Combat, The Psychology and Physiology of Deadly Conflict in War and in Peace (pg. 329)
Despite the proliferation of all things "tactical" among fitness programs in recent years, I think Lieutenant Colonel Dave Grossman is allowed... The following is an audio excerpt from "The Bullet Proof Mind" - from about 1:30 in, he discusses "tactical breathing" and then later emphasizes the wisdom of "In Victory, Tighten Your Helmet".