Looking intently with an unwavering gaze at a small point until tears are shed, is known as trataka by the acharyas.
("Hatha Yoga Pradipika" 2:31)
Trataka is a. deceptively simple but powerful practice. Trataka means 'to gaze steadily at a fixed point' and there are two forms of the practice. One is 'bahiranga' or external trataka and the other is 'antaranga' or internal trataka. Bahiranga is easier to practise because one simply has to gaze at an object or symbol. However, antaranga trataka involves clear and stable inner visualisation of an object.
In the practice of trataka an object is gazed at until its subtle form manifests in front of the closed eyes. The point of concentration is usually a symbol or object which activates the inner potential and can absorb the mind. The symbol most commonly used is a candle flame, because even after the eyes are closed, the impression remains naturally for some time, and then antaranga trataka can easily be performed. The purpose of focusing the eyes on an external object is to arouse the internal vision and make it absolutely steady by stopping the eye movements.