It is a minor Upaniṣad belonging to the Śukla Yajurveda and classed among the ‘Yoga-Upaniṣads,’ Upaniṣads that deal with yogic meditations and other processes. Except for the last part which is in verses (14 to 18), the whole Upaniṣad is in elegant prose.
It has been called ‘Advayatāraka’ since it deals with Brahman, the one without a second (‘advaya’), which takes us beyond (‘tāraka’) saṁsāra or transmigratory existence.
The Upaniṣad begins with the statement that it has to be taught to one who is struggling to meditate upon Brahman, the Absolute, (‘yati,’ ‘one who struggles’) who has practised self-control (‘jitendriya’) and who has cultivated the six virtues like peace (‘śamādiṣaṭka’). Then it describes that one who meditates upon light in between the eyebrows as Brahman attains to It. After negating the difference between the jīvātman (individual self) and Īśvara (Supreme Soul) as brought about by the limiting adjuncts like māyā, it goes on to describe a yogic meditation for attaining Brahman, the Absolute.
The suṣumnā-nāḍī (a tube-like structure meant for the passage of the kuṇḍalinī) (See KUṆḌALINĪ.) is situated in the middle of the body, stretching from the mulādhāra plexus at the root of the spinal column, up to the brahmarandhra or the aperture in the crown of the head. It shines like the sun or the moon. The Kuṇḍalinī which is fine like the thread in the lotus stalk, is inside this suṣumnā and is shining like millions of lightnings. By closing the ears and meditating on the sound within and the blue light within, between the eyes, and witnessing it, one gets infinite happiness. This is meditation on the ‘inside.’
The Upaniṣad then proceeds to describe the process of meditation on two points, ‘outside’ and ‘in between’ the ‘inside’ and the ‘outside.’ The first consists in fixing the gaze and the mind at any point in front of the nose, 4, 6, 8, 10 or 12 aṅgulas (an aṅgula is roughly about an inch) in distance, seeing it as of blue-yellow colour. The latter consists of meditating on the ‘ākāśa’ or ether/space, near the tip of the nose but beyond it, and as being bright like the full sun of the morning or fire with leaping tongues. In both cases this light becomes a symbol for Brahman.
Then comes the description of ‘tārakayoga,’ meditation on the light in the head, by reversing the mental light into the area between the eye brows, inside the head but towards the crown. Such meditation can give the supernatural ‘aṣṭasiddhis’ or ‘eight powers.’
Last comes the Śāmbhavīmudrā. It is the same as meditation already described, but, with the eyes without being closed or opened.
The Upaniṣad ends with a description of the qualifications of the guru fit to teach such yoga.