The Relaxation Response
In some ways, the fight-or-flight response is an all-or-nothing deal. All of the changes associated with fight-or-flight arousal are controlled by a special part of your nervous system called the sympathetic nervous system. Once a threat has passed, and your body begins to relax, another response, opposite to the fight-or-flight response kicks in. This is called the relaxation response, and it is governed by a completely separate system called the parasympathetic nervous system. The important thing to understand about these two systems is that they can not operate simultaneously. They are like two elevator operators, one that only knows how to go up, and one that only knows how to go down. Your body is the elevator, and only one operator is working at any given time.
If you consider the list of physical changes that are part of the fight-or-flight response, you'll notice that most of these are changes in physical functions to which you do not have direct access. Unless you are a practiced yogi, functions like heart rate, changes in blood flow and digestion, widening of the pupils, and perspiration are difficult if not impossible to consciously control. There are two physical processes involved in fight-or-flight arousal, however, that you do have varying degrees of access to: breathing and muscle tension.
Of the two, you are most easily able to manipulate your breathing. You can change the rate at which you breathe, whether you breathe shallowly or deeply, and whether you breathe through your mouth or through your nose. By manipulating how you breathe, it is possible to slow down and even reverse the fight-or-flight response.
To a lesser degree, you also have control over the level of tension in your muscles. For example, by making a fist, it is easy for most people to make the muscles in their arms tighter. What is more difficult, but not impossible, is to make your muscles more relaxed. With practice, it is possible to learn how to relax not only the muscles in your arms, but muscles in your shoulders, back, jaw, face, and other areas where chronic tension is a problem.