SB 4.12.17

तस्यां विशुद्धकरणः शिववार्विगाह्य
बद्ध्वासनं जितमरुन्मनसाहृताक्षः ।
स्थूले दधार भगवत्प्रतिरूप एतद्
ध्यायंस्तदव्यवहितो व्यसृजत्समाधौ ॥१७॥

tasyāṁ viśuddha-karaṇaḥ śiva-vār vigāhya
baddhvāsanaṁ jita-marun manasāhṛtākṣaḥ
sthūle dadhāra bhagavat-pratirūpa etad
dhyāyaṁs tad avyavahito vyasṛjat samādhau

tasyām—in Badarikāśrama; viśuddha—purified; karaṇaḥ—his senses; śiva—pure; vāḥ—water; vigāhya—bathing in; baddhvā—having fixed; āsanam—sitting position; jita—controlled; marut—breathing process; manasā—by the mind; āhṛta—withdrawn; akṣaḥ—his senses; sthūle—physical; dadhāra—he concentrated; bhagavat-pratirūpe—on the exact form of the Lord; etat—the mind; dhyāyan—meditating upon; tat—that; avyavahitaḥ—without stopping; vyasṛjat—he entered; samādhau—into trance.

In Badarikāśrama Dhruva Mahārāja's senses became completely purified because he bathed regularly in the crystal-clear purified water. He fixed his sitting position and by yogic practice controlled the breathing process and the air of life; in this way his senses were completely withdrawn. Then he concentrated his mind on the arcā-vigraha form of the Lord, which is the exact replica of the Lord and, thus meditating upon Him, entered into complete trance.

Here is a description of the aṣṭāṅga-yoga system, to which Dhruva Mahārāja was already accustomed. Aṣṭāṅga-yoga was never meant to be practiced in a fashionable city. Dhruva Mahārāja went to Badarikāśrama, and in a solitary place, alone, he practiced yoga. He concentrated his mind on the arcā-vigraha, the worshipable Deity of the Lord, which exactly represents the Supreme Lord, and thus thinking constantly of that Deity, he became absorbed in trance. Worship of the arcā-vigraha is not idol worship. The arcā-vigraha is an incarnation of the Lord in a form appreciable by a devotee. Therefore devotees engage in the temple in the service of the Lord as arcā-vigraha, a form made of sthūla (material) objects such as stone, metal, wood, jewels or paint. All of these are called sthūla, or physical representations. Since the devotees follow the regulative principles of worship, even though the Lord is there in His physical form, He is nondifferent from His original, spiritual form. Thus the devotee gets the benefit of achieving the ultimate goal of life, that is to say, becoming always absorbed in thought of the Lord. This incessant thought of the Lord, as prescribed in the Bhagavad-gītā, makes one the topmost yogī.