abhyasen manasā śuddhaṁ
mano yacchej jita-śvāso
abhyaset—one should practice; manasā—by the mind; śuddham—sacred; tri-vṛt—composed of the three; brahma-akṣaram—transcendental letters; param—the supreme; manaḥ—mind; yacchet—get under control; jita-śvāsaḥ—by regulating the breathing air; brahma—absolute; bījam—seed; avismaran—without being forgotten.
After sitting in the above manner, make the mind remember the three transcendental letters [a-u-m], and by regulating the breathing process, control the mind so as not to forget the transcendental seed.
Oṁkāra, or the praṇava, is the seed of transcendental realization, and it is composed of the three transcendental letters a-u-m. By its chanting by the mind, in conjunction with the breathing process, which is a transcendental but mechanical way of getting into trance, as devised by the experience of great mystics, one is able to bring the mind, which is materially absorbed, under control. This is the way of changing the habit of the mind. The mind is not to be killed. Mind or desire cannot be stopped, but to develop a desire to function for spiritual realization, the quality of engagement by the mind has to be changed. The mind is the pivot of the active sense organs, and as such if the quality of thinking, feeling and willing is changed, naturally the quality of actions by the instrumental senses will also change. Oṁkāra is the seed of all transcendental sound and it is only the transcendental sound which can bring about the desired change of the mind and the senses. Even a mentally deranged man can be cured by treatment of transcendental sound. In the Bhagavad-gītā, the praṇava (oṁkāra) has been accepted as the direct, literal representation of the Supreme Absolute Truth. One who is not able to chant directly the holy name of the Lord, as recommended above, can easily chant the praṇava (oṁkāra). This oṁkāra is a note of address, such as "O my Lord," just as oṁ hari om means "O my Lord, the Supreme Personality of Godhead." As we have explained before, the Lord's holy name is identical with the Lord Himself. So also is oṁkāra. But persons who are unable to realize the transcendental personal form or name of the Lord on account of their imperfect senses (in other words, the neophytes) are trained to the practice of self-realization by this mechanical process of regulating the breathing function and simultaneously repeating the praṇava (oṁkāra) within the mind. As we have several times expressed, since the transcendental name, form, attributes, pastimes, etc., of the Personality of Godhead are impossible to understand with the present material senses, it is necessary that through the mind, the center of sensual activities, such transcendental realization be set into motion. The devotees directly fix their minds on the Person of the Absolute Truth. But one who is unable to accommodate such personal features of the Absolute is disciplined in impersonality to train the mind to make further progress.
Link to this page: https://prabhupadabooks.com/sb/2/1/17