SB 4.12.17
tasyam visuddha-karanah siva-var vigahya
baddhvasanam jita-marun manasahrtaksah
sthule dadhara bhagavat-pratirupa etad
dhyayams tad avyavahito vyasrjat samadhau
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In Badarikasrama Dhruva Maharaja’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and by yogic practice controlled the breathing process and the air of life; in this way his senses were completely withdrawn.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 arca-vigraha form of the Lord, which is the exact replica of the Lord and, thus meditating upon Him, entered into complete trance.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Here is a description of the astanga-yoga system, to which Dhruva Maharaja was already accustomed. Astanga-yoga was never meant to be practiced in a fashionable city. Dhruva Maharaja went to Badarikasrama, and in a solitary place, alone, he practiced yoga. He concentrated his mind on the arca-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 arca-vigraha is not idol worship. The arca-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 arca-vigraha, a form made of sthula (material) objects such as stone, metal, wood, jewels or paint. All of these are called sthula, 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-gita, makes one the topmost yogi.