Lord Śrī Caitanya Mahāprabhu bestows His causeless mercy upon His devotees in three features: His own direct appearance (sākṣāt), His prowess within someone He empowers (āveśa), and His manifestation (āvirbhāva).
The sākṣāt feature of Śrī Caitanya Mahāprabhu is His personal presence. Āveśa refers to invested power, like that invested in Nakula Brahmacārī. Āvirbhāva is a manifestation of the Lord that appears even though He is personally not present. For example, Śrī Śacīmātā offered food at home to Śrī Caitanya Mahāprabhu although He was far away in Jagannātha Purī, and when she opened her eyes after offering the food, she saw that it had actually been eaten by Śrī Caitanya Mahāprabhu. Similarly, when Śrīvāsa Ṭhākura performed saṅkīrtana, everyone felt the presence of Śrī Caitanya Mahāprabhu, even in His absence. This is another example of āvirbhāva.
The appearance of Lord Śrī Caitanya Mahāprabhu in every devotee's presence is called sākṣāt. His appearance in Nakula Brahmacārī as a symptom of special prowess is an example of āveśa.
In his body there were symptoms of āvirbhāva. Such appearances are uncommon, but Lord Caitanya Mahāprabhu displayed many such pastimes through His different features.
In the Gaura-gaṇoddeśa-dīpikā (73–74) it is said that Nakula Brahmacārī displayed the prowess (āveśa) and Pradyumna Brahmacārī the appearance (āvirbhāva) of Śrī Caitanya Mahāprabhu. There are many hundreds and thousands of devotees of Lord Caitanya among whom there are no special symptoms, but when a devotee of Lord Śrī Caitanya Mahāprabhu functions with specific prowess, he displays the feature called āveśa. Śrī Caitanya Mahāprabhu personally spread the saṅkīrtana movement, and He advised all the inhabitants of Bhāratavarṣa to take up His cult and preach it all over the world. The visible bodily symptoms of devotees who follow such instructions are called āveśa. Śrīla Śivānanda Sena observed such āveśa symptoms in Nakula Brahmacārī, who displayed symptoms exactly like those of Śrī Caitanya Mahāprabhu. The Caitanya-caritāmṛta states that in the Age of Kali the only spiritual function is to broadcast the holy name of the Lord, but this function can be performed only by one who is actually empowered by Lord Kṛṣṇa. The process by which a devotee is thus empowered is called āveśa, or sometimes it is called śakty-āveśa.
Śrī Caitanya Mahāprabhu delivered almost all the fallen souls by directly meeting them. He delivered others by entering the bodies of great devotees, such as Nakula Brahmacārī. And He delivered still others by appearing before them, as in the case of Nṛsiṁhānanda Brahmacārī. "I shall deliver the fallen souls." This statement characterizes the Supreme Personality of Godhead.
In Āmbuyā-muluka there was a person named Nakula Brahmacārī, who was a perfectly pure devotee, greatly advanced in devotional service.
Śrīla Bhaktivinoda Ṭhākura says that Āmbuyā-muluka is the present Ambikā, a city in the Vardhamāna district of West Bengal. Formerly, during the Muslim regime, it was known as Āmbuyā-muluka. In this city there is a neighborhood called Pyārīgañja, and that is where Nakula Brahmacārī used to live.
Desiring to deliver all the people of Bengal, Śrī Caitanya Mahāprabhu entered the heart of Nakula Brahmacārī.
Nakula Brahmacārī became exactly like a man haunted by a ghost. Thus he sometimes laughed, sometimes cried, sometimes danced and sometimes chanted like a madman.
When Śivānanda Sena heard that Śrī Caitanya Mahāprabhu had entered the body of Nakula Brahmacārī, he went there with doubts in his mind.
Desiring to test the authenticity of Nakula Brahmacārī, he stayed outside, thinking as follows.
"If Nakula Brahmacārī personally calls me and knows my worshipable mantra, then I shall understand that he is inspired by the presence of Śrī Caitanya Mahāprabhu." Thinking in this way, he stayed some distance apart.
There was a large crowd of people, some coming and some going. Indeed, some people in that great crowd could not even see Nakula Brahmacārī.
In his inspired state, Nakula Brahmacārī said, "Śivānanda Sena is staying some distance away. Two or four of you go call him."
Thus people began running here and there, calling in all directions, "Śivānanda! Whoever is Śivānanda, please come. Nakula Brahmacārī is calling you."
Hearing these calls, Śivānanda Sena quickly went there, offered obeisances to Nakula Brahmacārī, and sat down near him.
Nakula Brahmacārī said, “I know that you are doubtful. Now please hear this evidence with great attention.
Śivānanda Sena thereupon developed full confidence in his mind that Nakula Brahmacārī was filled with the presence of Śrī Caitanya Mahāprabhu. Śivānanda Sena then offered him respect and devotional service.