Srimad Bhagavatam Canto 04, Chapter 12, Text 18

SB 4.12.18

bhaktim harau bhagavati pravahann ajasram
ananda-baspa-kalaya muhur ardyamanah
viklidyamana-hrdayah pulakacitango
natmanam asmarad asav iti mukta-lingah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
Because of his transcendental bliss, incessant tears flowed from his eyes, his heart melted, and there was shivering and standing of the hairs all over his body. Thus transformed, in a trance of devotional service, Dhruva Maharaja completely forgot his bodily existence, and thus he immediately became liberated from material bondage.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
Due to constant engagement in devotional service — hearing, chanting, remembering, worshiping the Deity, etc., as prescribed in nine varieties — there are different symptoms which appear in the body of a devotee. These eight bodily transformations, which indicate that a devotee is already liberated within himself, are called asta-sattvika-vikara. When a devotee completely forgets his bodily existence, he should be understood to be liberated. He is no longer encaged in the body. The example is given that when a coconut becomes completely dry, the coconut pulp within the coconut shell separates from the bondage of the shell and the outer covering. By moving the dry coconut, one can hear that the pulp within is no longer attached to the shell or to the covering. Similarly, when one is fully absorbed in devotional service, he is completely disconnected from the two material coverings, the subtle and gross bodies. Dhruva Maharaja actually attained this stage of life by constantly discharging devotional service. He has already been described as a maha-bhagavata, for unless one becomes a maha-bhagavata, or a first-class pure devotee, these symptoms are not visible. Lord Caitanya exhibited all these symptoms. Thakura Haridasa also exhibited them, and there are many pure devotees who manifested such bodily symptoms. They are not to be imitated, but when one is actually advanced, these symptoms are exhibited. At that time it is to be understood that a devotee is materially free. Of course, from the beginning of devotional service the path of liberation immediately opens, just as the coconut taken from the tree immediately begins to dry; it simply takes some time for the shell and pulp to separate from one another.
An important word in this verse is mukta-lingah. Mukta means “liberated,” and linga means “the subtle body.” When a man dies, he quits the gross body, but the subtle body of mind, intelligence and ego carries him to a new body. While existing in the present body, the same subtle body carries him from one stage of life to another (for example, from childhood to boyhood) by mental development. The mental condition of a baby is different from that of a boy, the mental condition of a boy is different from that of a young man, and the mental condition of a young man is different from that of an old man. So at death the process of changing bodies takes place due to the subtle body: the mind, intelligence and ego carry the soul from one gross body to another. This is called transmigration of the soul. But there is another stage, when one becomes liberated even from the subtle body; at that time the living entity is competent and fully prepared to be transferred to the transcendental or spiritual world.
The description of the bodily symptoms of Sri Dhruva Maharaja makes it apparent that he became perfectly fit to be transferred to the spiritual world. One can experience the distinction between the subtle and gross bodies even daily: in a dream, one’s gross body is lying on the bed while the subtle body carries the soul, the living entity, to another atmosphere. But because the gross body has to be continued, the subtle body comes back and settles in the present gross body. Therefore one has to become free from the subtle body also. This freedom is known as mukta-linga.
Srimad Bhagavatam Canto 04, Chapter 12, Text 17
Srimad Bhagavatam Canto 04, Chapter 12, Text 19