The sympathetic nervous system uses the vital energy. There are two systems-left and right. The two channels, which carry this energy, in subtle form (in the medulla oblongata), are known as Ida and Pingala respectively. The right-side system (in the right-handed person) caters to the emergencies of the active consciousness (extra efforts and emergencies). The left-side system (in medicine they say it remains dormant) caters to the subconscious mind of the psyche (libido).
Both these sympathetic-nerve systems are called the Sun and Moon channels (i.e., Surya nadi and Chandra nadi). Hatha Yoga comes from the words Ha and Tha, meaning the Sun and Moon. By this yoga you can control the activity of the sympathetic; you can use more stored energy or else completely stop the activity of the sympathetic for a short time. Just by using the stored energy you cannot achieve the eternal flow of the Divine. With control over the sympathetic you can slow the heart rate or even stop the heart for a short time. You may achieve all the physical gross symptoms of the parasympathetic. But you cannot activate the parasympathetic which is the channel for your real yoga (meeting of the Divine). With Hatha Yoga you may even control your mind. But the mind thus governed is heavily conditioned for the spiritual flight into the divine. One may keep good health and good mind to be a good citizen in a society, but that is not the only aim of life.