I practiced the “no blinking” exercise with two students the other night. The idea is to fix you attention on a small object or here on the top of a candle flame and try not to blink for a few breaths, getting the mind completely involved in the small flame. When you are about to blink again, close your eyes and watch for the light to reappear in the dark
Sit down and place a lighted candle about two feet in front of you with the flame at eye level. Gaze at the middle of the flame until your eyes water, internal Trataka can then be performed, by closing the eyes and allowing the image of the flame to appear. Try to keep the image clear and unwavering. Repeat the process until you can hold the image externally without blinking as well as internally, without wavering.
Physiologically, Trataka “cures” diseases of the eye such as eyestrain, headache, astigmatism, and myopia. The eyes become clear and bright and able to see the reality beyond external appearances.
Psychologically, Trataka develops clairvoyance, telepathy, and telekinesis as well as strong will power and ekagrata, meaning single pointedness, without which concentration and meditation are not possible.
The Hatha Yoga practitioner uses the purified and tuned instrument of the body in order to gain true perception of reality. Swami Muktibodhananda writes in the Bihar School commentary on the Hatha Yoga Pradipika* that vision depends not only on the organs of the eyes, which are lenses or mediums for external perception but on the entire optic tracks. When you look at something, an image is projected onto the retina via the eyes, which stimulates the retina to fire impulses back to the visual cortex of the brain where an inner image is mapped out. When the image of the external object is stabilized on the retina, and held there for some time, without wavering, then the image will completely disappear and along with it a suspension of normal mental processes; in other words the mind will be turned off. More info here