Śrī Rūpa Gosvāmī has defined rāgānugā-bhakti as spontaneous attraction for something while completely absorbed in thoughts in it, with an intense desire of love. Devotional service executed with such feelings of spontaneous love is called rāgānugā-bhakti. Devotional service under the heading of rāgānugā can be further divided into two categories: one category is called "sensual attraction," and the other is called "relationship."
(Nectar of Devotion).

The mood of surrender during the stage of vaidhi-bhakti (devotional service under strict rules and regulations) is different from that in the stage ofrāgānuga-bhakti (spontaneous devotional service). In the spontaneous stage, the mood of surrender is the natural expression of the self. When the process of surrender is followed step by step, one patiently executes the Lord's orders and gradually becomes enthusiastic. Such a devotee follows the regulative principles of hearing, chanting, remembering, and so on, and emulates previous saintly preceptors. In the association of devotees he becomes more and more proficient in rendering devotional service. Gradually his service becomes easier. Thus constant remembrance of the Lord comes simply by developing enthusiasm and patience in devotional activities.
(Renunciation Through Wisdom).

An advanced devotee is attracted by the service rendered by an eternal servitor of the Lord. This attraction is called spontaneous attraction. Technically it is called svarūpa-upalabdhi. This stage is not achieved in the beginning. In the beginning one has to render service strictly according to the regulative principles set forth by the revealed scriptures and the spiritual master. By continuously rendering service through the process of vaidhī bhakti, one's natural inclination is gradually awakened. That is called spontaneous attraction, or rāgānugā bhakti.
(Sri Caitanya Caritamrta--2:22:153--purport).

When one has developed such attachment, he can spontaneously serve the lotus feet of Kṛṣṇa even without following the regulative principles. This stage is called rāga-bhakti, or devotional service in spontaneous love. At that stage the devotee can follow in the footsteps of one of the eternal associates of Kṛṣṇa in Vṛndāvana. This is called rāgānuga-bhakti. Rāgānuga-bhakti, or spontaneous devotional service, can be executed in the śānta-rasa when one aspires to be like Kṛṣṇa's cows or the stick or flute in the hand of Kṛṣṇa, or the flowers around Kṛṣṇa's neck. In the dāsya-rasa one follows in the footsteps of servants like Citraka, Patraka or Raktaka. In the friendly sakhya-rasa one can become a friend like Baladeva, Śrīdāmā or Sudāmā. In the vātsalya-rasa, characterized by parental affection, one can become like Nanda Mahārāja and Yaśodā, and in the mādhurya-rasa, characterized by conjugal love, one can become like Śrīmatī Rādhārāṇī or Her lady friends such as Lalitā and Her serving maids (mañjarīs) like Rūpa and Rati. This is the essence of all instruction in the matter of devotional service."
(Nectar of Instruction--purport).