After passing through the six centres, the Kundalini reaches the Thousand-Petalled Lotus known as the Sahasrara, and the aspirant goes into Samadhi.
According to the Vedas these centres are called 'bhumi', 'planes'. There are seven such planes. The centre at the heart corresponds to the fourth plane of the Vedas. According to the Tantra there is in this centre a lotus called Anahata, with twelve petals.
The centre known as the Visuddha is the fifth plane. This centre is at the throat and has a lotus of sixteen petals. When the Kundalini reaches this plane, the devotee longs to talk and hear only about God. Conversation on worldly subjects, on 'woman and gold', causes him great pain. He leaves a place where people talk of these matters.
Then comes the sixth plane, corresponding to the centre known as Ajna. This centre is located between the eyebrows and it has a lotus with two petals. When the Kundalini reaches it, the aspirant sees the form of God. But still there remains a slight barrier between the devotee and God. It is like a light inside a lantern. You may think you have touched the light, but in reality you cannot because of the barrier of the glass.
And last of all is the seventh plane, which, according to Tantra, is the centre of the Thousand-Petalled Lotus. When the Kundalini arrives there, the aspirant goes into Samadhi. In that lotus dwells Satchitananda Siva, the Absolute. There Kundalini, the awakened Power, unites with Siva. This is known as the union of Siva and Sakti.
When the Kundalini rises to the Sahasrara and the mind goes into samdhi, the aspirant loses all consciousness of the outer world. He can no longer remain in his physical body. If milk is poured into his mouth, it runs out again. In that state the life-breath lingers for twenty-one days and then passes out. Entering the 'black waters' of the ocean, the ship never comes back. But the Isvarakotis, such as the Incarnations of God, can come down from this state of samadhi. They can descend from this exalted state because they like to live in the company of devotees and enjoy the love of God. God retains in them the 'ego of Knowledge' or the 'ego of Devotion' so that they may teach men. Their minds move between the sixth and the seventh planes. They run a boat race back and forth, as it were, between these two planes.
After attaining Samadhi some souls of their own accord keep the 'ego of Knowledge'. But that ego does not create any attachments. It is like a line drawn on water."
(The Gospel of Sri Ramakrishna by Swami Nikhilananda. pp. 449-500)