[The vṛtti states of mind] are stilled by practice and dispassion.

Bryant Commentary:

At the beginning of the text (I.2), Patañjali defined yoga as citta-vṛtti-nirodha, the restraint of the vṛttis, the changing states of the mind. Having explained what constitutes a vṛtti, he now turns his attention to nirodha, restraint. How, exactly, are the vṛttis to be restrained? In this sūtra he identifies two ingredients necessary for such restraint: practice and dispassion (renunciation). Vijñānabhikṣu quotes the Gītā here: “The mind is undoubtedly fickle and difficult to control, O Arjuna, but it can be controlled by abhyāsa–vairāgyābhyām, practice and dispassion” (VI.35). The same two ingredients are indicated by Patañjali in this sūtra. As was seen in I.6, Patañjali’s typical method is to introduce a list in one sūtra and then explain the items on this list in the subsequent sūtras, so his definitions of practice and dispassion follow in the next sūtras.

Before proceeding to an analysis on practice and dispassion, Vyāsa notes that the stream of the citta, mind, can flow two ways: toward its upliftment or toward its downfall. He analogizes the mind to a river, which normally flows down the channels of the senses toward their objects and into the sea of saṁsāra, the cycle of birth and death. However, by dispassion toward the sense objects, the flow of this river of the mind toward the sea of saṁsāra is checked, and by discrimination, the current of the river is reversed and the mind flows back, away from saṁsāra, and toward realization of the self. By flowing along the course of discrimination, the mind leads to upliftment and ultimate liberation; contrarily, if it flows along the course of nondiscrimination in the form of sensuality, it produces karma, which may be good or bad depending on whether the actions the mind provokes are pious or impious, and perpetuates the vicious cycle of repeated birth and death. By practice and dispassion, the flow of the mind toward sensual attractions, which might entice the mind toward vice, becomes drastically diminished. Rather, by practice, which of course refers to the practice of yoga, the flow of the mind toward higher knowledge becomes unobstructed, and the mind becomes immersed in discrimination.

Another way of putting this from the perspective of the guṇas is that sattva becomes enhanced and rajas and tamas minimized. Discrimination, dispassion, and the impetus to seek a practice in order to realize Truth are inherent in the mind when its sāttvic potential is not overwhelmed by rajas and tamas, which are the influences provoking the flow of the mind toward sensuality. Bhoja Rāja adds that by practice and renunciation, eventually all fluctuating states of the mind, whether sāttvic, rājasic, or tāmasic, can be controlled. He understands dispassion as the realization by the wise of the negative repercussions of sensuality, which results in avoidance of it—the pursuit of sensual pleasure always bears a hidden price.

 By Practice And Detachment These Can Be Stopped.

Both practice and non-reaction are required to still the patterning of consciousness.

The vacillating waves of perceptions are stilled through consistent earnest practice and dispassionate non-attachment.

Their suppression (is brought about) by persistent practice and nonattachment.

Taimini Commentary:

 After classifying and explaining the different forms which modifications of the mind may assume, the author gives in this Sutra the two general means of bringing about the suppression of these modifications. These are practice and non-attachment. Two apparently simple words, but what a tremendous effort of the human will and variety of practices they stand for! Both these words have been defined in the subsequent Sutras but their full significance can be understood only after the study of the book has been completed.

Practice and detachment are the means to still the movements of consciousness.

Iyengar Commentary:

The fluctuations of consciousness, painful or non-painful, described in 1.5 and 1.6, are to be controlled through repeated yogic practice. Mental strength must also be developed, to attain detachment and freedom from desires.

Study of the consciousness and stilling it is practice (abhyasa). Elsewhere (11.28) Patañjali has used another word: anusthana. Abhyasa conveys the sense of mechanical repetition, whereas anusthana implies devotion, dedication, a religious attitude. Repeated effort made with a thorough understanding of the art and philosophy of yoga and with perfect communion of body, mind and soul is not a mechanical practice but a religious and spiritual one.

Practice is the positive aspect of yoga; detachment or renunciation (vairagya) the negative. The two balance each other like day and night, inhalation and exhalation. Practice is the path of evolution; detachment and renunciation the path of involution. Practice is involved in all the eight limbs of yoga. Evolutionary practice is the onward march towards discovery of the Self, involving yama, niyama, Asana and pranayama. The involutionary path of renunciation involves pratyahara, dharana, dhyana and samadhi. This inward journey detaches the consciousness from external objects.

Patañjali’s practice represents the ha or ‘sun’ aspect, and renunciation the tha or ‘moon’ aspect of hatha yoga. In hatha yoga, ha represents the life-force and tha, the consciousness. Ha also represents the very being – the seer, while tha is the reflected light of the seer, representing citta. Through Hathay-yoga these two forces are blended, and then merged in the seer.

To be adept in yoga, yama and niyama must be observed carefully throughout the yogic sadhana. This is abhyasa. The discarding of ideas and actions which obstruct progress in sadhana is vairagya.

As we know, consciousness becomes involved with the objects seen, and identifies with them, drawing the seer with it. Then the seer becomes subordinate to the oscillating mind. The eight aspects of yoga, described in 11.29, are given to us as a means to stop the wavering of the intelligence and to learn correct understanding. Although the first four relate to practice and the others to renunciation, practice and renunciation are interdependent and equally important. Without restraint, the forces generated by practice would spin out of control and could destroy the sadhaka. At the higher levels, vairagya without abhyasa could lead to stagnation and inner decay. The first four aspects are considered a building up process, and the last four one of inner consolidation. Once our initial tamasic nature moves towards a dynamic state, restraint becomes necessary for our own inner security.

Vairagya is a practice through which the sadhaka learns to be free from desires and passions and to cultivate non-attachment to things which hinder his pursuit of union with the soul.

The disciplines which are to be followed are explained in the succeeding sutras.
(For yogic disciplines see 11.29–32, 11.35–53.)

These mental modifications are restrained by practice and non-attachment.

Satchidananda Commentary:

 On the positive side, you practice. On the other side, you detach yourself from the cause for these modifications. Patañjali gives both a positive and negative approach to thought control, which he proceeds to elucidate in the following sūtras.

Their control is by practice and non-attachment.

SV Commentary:

The mind, to have this non-attachment, must be clear, good and rational. Why should we practice? Because each action is like the pulsations quivering over the surface of the lake. The vibration dies out, and what is left? The Samsharas, the impressions. When a large number of these impressions is left on the mind they coalesce, and become a habit. It is said “habit is second nature;” it is first nature also, and the whole nature of man; everything that we are, is the result of habit. That gives us consolation, because, if it is only habit, we can make and unmake it at any time. The Samshara is left by these vibrations passing out of our mind, each one of them leaving its result. Our character is the sum-total of these marks, and according as some particular wave prevails one takes that tone. If good prevail one becomes good, if wickedness one wicked, if joyfulness one becomes happy. The only remedy for bad habits is counter habits; all the bad habits that have left their impressions are to be controlled by good habits. Go on doing good, thinking holy thoughts continuously; that is the only way to suppress base impressions. Never say any man is hopeless, because he only represents a character, a bundle of habits, and these can be checked by new and better ones. Character is repeated habits, and repeated habits alone can reform character. 

Well then, what is the means for restraining them ? “ They are restrained by practice and desirelessness.”

The stream of mind flows both ways ; flows towards good and it flows towards evil. That which flows on to perfect independence (kaivalya) down the plane of discriminative- knowledge, is named the stream of happiness. That which leads to re-birth and flows down the plane of undiscriminative ignorance, is the stream of sin.

Among these the flow of the desirables is thinned by desirelessness ; the flow of discrimination is rendered visible by habituating the mind to the experience of knowledge.

Hence suppression of the mental modification is dependent upon both.

~ Rāma Prasāda translation.