And, O mighty ascetic, remaining in the midst of the Milky sea in the shape of a tortoise, the revered Hari himself became the support of the churning-stick. And the holder of the discus and mace, remaining in one form in the midst of the immortals, and in another, in the midst of the Daityas,—began to pull the king of serpents. And, O Maitreya, in another gigantic form, Keçava pulled the mountain upwards,—which form was seen neither of the celestials nor the Asuras.

In plain words: Vishnu took the form of a giant tortoise and held up the mountain from below in the middle of the ocean. He also took one form among the gods and another among the demons to help pull the serpent. In yet another huge form, which no one could see, he held the mountain from above.