Yogamāyā is the principal potency of the Personality of Godhead. In the Vedas it is stated that the Lord, the Supreme Personality of Godhead, has multipotencies: parāsya śaktir vividhaiva śrūyate (CC Madhya 13.65, purport). All the different potencies are acting externally and internally, and Yogamāyā is the chief of all potencies. The Lord ordered the appearance of Yogamāyā in the land of Vrajabhūmi, in Vṛndāvana, which is always decorated and full with beautiful cows. In Vṛndāvana, Rohiṇī, one of the wives of Vasudeva, was residing at the house of King Nanda and Queen Yaśodā. Not only Rohiṇī but many others in the Yadu dynasty were scattered all over the country due to their fear of the atrocities of Kaṁsa. Some of them were even living in the caves of the mountains.
The Lord thus informed Yogamāyā: “Under the imprisonment of Kaṁsa are Devakī and Vasudeva, and at the present moment My plenary expansion Śeṣa is within the womb of Devakī. You can arrange the transfer of Śeṣa from the womb of Devakī to the womb of Rohiṇī. After this arrangement, I am personally going to appear in the womb of Devakī with My full potencies. Then I shall appear as the son of Devakī and Vasudeva. And you shall appear as the daughter of Nanda and Yaśodā in Vṛndāvana.
“Since you will appear as My contemporary sister, and since you will quickly satisfy desires for sense gratification, people within the world will worship you with all kinds of valuable presentations: incense, candles, flowers and offerings of sacrifice. People who are after materialistic perfection will worship you under the different forms of your expansions, which will be named Durgā, Bhadrakālī, Vijayā, Vaiṣṇavī, Kumudā, Caṇḍikā, Kṛṣṇā, Mādhavī, Kanyakā, Māyā, Nārāyaṇī, Īśānī, Śāradā and Ambikā.”
Kṛṣṇa and Yogamāyā appeared as brother and sister—the Supreme Powerful and the supreme power. Although there is no clear distinction between the Powerful and the power, power is always subordinate to the Powerful. Those who are materialistic are worshipers of the power, but those who are transcendentalists are worshipers of the Powerful. Kṛṣṇa is the Supreme Powerful, and Durgā is the supreme power within the material world. Actually people in the Vedic culture worship both the Powerful and the power. There are many hundreds and thousands of temples of Viṣṇu and Devī, and sometimes they are worshiped simultaneously. The worshiper of the power, Durgā, or the external energy of Kṛṣṇa, may achieve all kinds of material success very easily, but anyone who wants to be elevated transcendentally must engage in worshiping the Powerful in Kṛṣṇa consciousness.
The Lord also declared to Yogamāyā, “My plenary expansion Ananta Śeṣa is within the womb of Devakī. On account of being forcibly attracted to the womb of Rohiṇī, He will be known as Saṅkarṣaṇa and will be the source of all spiritual power, or bala, by which one can attain the highest bliss of life, which is called ramaṇa. Therefore the plenary portion Ananta will be known after His appearance either as Saṅkarṣaṇa or as Balarāma.”
In the Upaniṣads it is stated, nāyam ātmā bala-hīnena labhyaḥ. The purport is that one cannot attain the supreme platform of self-realization without being sufficiently favored by Balarāma. Bala does not mean physical strength. No one can attain spiritual perfection by physical strength. One must have the spiritual strength which is infused by Balarāma, or Saṅkarṣaṇa. Ananta, or Śeṣa, is the source of the power which sustains all the planets in their different positions. Materially this sustaining power is known as the law of gravitation, but actually it is a display of the potency of Saṅkarṣaṇa. Balarāma, or Saṅkarṣaṇa, is the source of spiritual power, or the original spiritual master. Therefore Lord Nityānanda Prabhu, who is also the incarnation of Balarāma, is the original spiritual master. And the spiritual master is the representative of Balarāma, the form of the Supreme Personality of Godhead who supplies spiritual strength. In the Caitanya-caritāmṛta it is confirmed that the spiritual master is the manifestation of the mercy of Kṛṣṇa.