Chapter 18, Verse 33
Part of 18: Mokṣha Sanyās Yog — मोक्षसंन्यासयोगधृत्या यया धारयते मनःप्राणेन्द्रियक्रियाः।योगेनाव्यभिचारिण्या धृतिः सा पार्थ सात्त्विकी॥
dhṛityā yayā dhārayate manaḥ-prāṇendriya-kriyāḥ yogenāvyabhichāriṇyā dhṛitiḥ sā pārtha sāttvikī
The unwavering firmness, through which Yoga restrains the functions of the mind, life-force, and senses—that firmness, O Arjuna, is Sattvic (pure).
When firmness is awakened in the mind, the activities of the mind, the lifeforce and the senses are brought under control. The senses are withdrawn into the mind. The Prana and the Apana pass into the Sushumna Nadi. Yoga Samadhi or concentration of the mind. You cannot restrain the mind, the lifeforce and the senses by mere firmness. You can control them only by firmness which is ever accompanied by concentration of the mind. When the mind, the lifeforce and the senses are curbed by unwarvering firmness, they cannot run towards external sensual objects, they cannot do any mischief, they cannot move in ways which are opposed to the scriptures, they will be absorbed into their respective causes and their outgoing tendencies will be totally checked. This firmness is not repression or suppression, but an intelligent sublimation and inner transformation.