From Ishavasya Upanishad, from Swami Niranjanananda Saraswati
The word 'Guru' has been used in the scriptures in two meanings. One is the Guru who removes the darkness, and the other is the Guru from whom we receive the mantra of initiation. The Guru is the one who guides us in our spiritual practice. There is an important chakra in the body which is known as Ajna Chakra or Guru Chakra. It is located at the top of the spine, behind the eyebrows center, where Ida, Pingala and Sushumna merge into one channel.
The Ajna chakra has a unique quality: in the process of cultivation, as the external awareness of cultivation becomes dormant, this chakra becomes progressively and proportionately active. On the one hand, the functions of the senses, the psyche, Buddha, and Chitta are withdrawn, and on the other hand, the form of the Ashta deity, or individual deity, becomes clear as well as the chakshu awakens. As soon as the senses become extroverted, the cycle of ignorance becomes dormant. The awakening of the Anjan Chakra occurs during meditation only in those seekers whose senses are withdrawn and whose brain is largely calm and isolated.
There are some seekers whose Aja Chakra remains awake even while meditating and even when not in meditation. However, such people are rare. In the state of deep meditation, when all the senses are withdrawn, the Guru's subtle instructions are received. Also, all the peeks of perception are closed in this state. Between the macro and micro states there is a membrane that acts as a soundproofing device. When you are in a gross state, you do not hear subtle sounds, and when you are crossed in a subtle state, you cannot hear gross sounds. Experiences in one area do not enter into another. The senses are able to feel only specific vibrations within a particular range, even though the sound waves of all frequencies are continuously present.
Instruction in deep meditation does not originate from thought, as thought is a product of the mind, which is gross. The Guru instructs the disciple through a subtle form of transmission. Only those Gurus who are able to remain in the subtle state can give subtle instructions. However, it is very difficult to stay for a long time in a subtle state, where no karma or activity can take place. Two swords cannot be placed in a sheath. To remain in this state all actions have to be dropped, whether direct or indirect. It is necessary to release all worldly attachments, erase all effects of pleasure and pain, rise above the stretch of senses, mind and intellect, and there is no difference between auspicious and inauspicious. This part of Guru is rarely available.
When such a Guru transmits instructions to his disciple during meditation, a counter vibration is produced in the disciple's Anjan Chakra which leads him further on the cultivation path. The moment the arrow of the Guru's instruction strikes the disciple, he feels as if he has crossed from a small dark room into a vast space. This place does not appear as a special world of memories and dreams; It is exactly like a gross world in appearance, with all the same assumptions. This inner world is similar to the outer world with only one difference: the outer world is perishable, while the inner world is eternal. After arriving at the inner world, the outer world appears to be false.
This experience is possible only when the Anjana Chakra is open and the Guru's command is heard there. After this stage, sadhana does not perform any other practice except following the Guru's words. Up to this point, he performs various sadhanas to awaken the Anjana Chakra, but beyond this point, only the Guru's transmission can lead him forward. Moving forward, he considers the light of the self and realizes that he and the self are one. There is no difference between the two. This is the incident of knowing you - "On knowing you, I become you". This is the goal of all yogas and practices.