Trāṭaka (Sanskrit n. त्राटक trāṭaka, tratak, trataka: "to look, or to gaze") is the practice of staring at some external object. This fixed gazing is a method of meditation which involves concentrating on a single point such as a small object, black dot or candle flame. It is used in yoga as a way of developing concentration, strengthening the eyes, and stimulating the ājňā chakra.
With the ability to cease the restlessness of eyes seeking something or the other, by fixing the gaze, the restless mind too comes to a halt. Trāṭaka is said to enhance the ability to concentrate. It may increase the power of memory and bring the mind in a state of awareness, attention and focus. This exercise acts on the centers of smell and sight; it stimulates the nervous system. It is said to control the ciliary (blink) reflex and stimulate the pineal gland.
Tratak is practised in two stages. In the first, the practitioner fixes attention on a symbol or yantra, such as the Om symbol, a black dot, or the image of some deity, and stares at it, paying attention to each thought and feeling as it arises, and letting them go, so that the mind is completely absorbed with the symbol. The practice continues until the eyes begin to water, at which point they are closed, and relaxed.
The second stage is staring at a candle flame. The practice is the same up until the eyes begin to water, after which the eyes are closed, and the yogi tries to concentrate on the after image, and hold it for as long as possible. At first, it will be a real after image, but later, it will exist only in the mind's eye, and the exercise in concentration comes from trying to hold it there for a long period of time.
Sample guided meditation
Stage 1 Light a candle, three to four feet (1 metre plus) away. Sit in front of it. The flame should be at the level of the eyes so that it can be seen straight without being uncomfortable. Begin with slow and deep breathing. While breathing in this manner, steadily gaze at the flame. Keep the spine erect. Keep the gaze fixed on the flame without being distracted by outer disturbances or thoughts. If thoughts arise, simply ignore them; do not struggle to remove them. It is important to be wakeful and vigilant.
Stage 2 Imagine the flame is entering your body through the eyes and illuminating your inner being. At this stage, let the eyes close with ease. Now, try to imagine the same flame with closed eyes as you were seeing it with eyes open. If you are able to practice tratak without blinking, it will be easy for you to "see" the flame with closed eyes.
The Bihar School of Yoga, in India has published several books on meditation that give detailed instructions for practising Trataka. Dharana Darshan by Swami Niranjanananda Saraswati has an entire chapter devoted to the practice.