Being thus reminded of his real character by Rāma, Krishna smiled gently and immediately freed himself from the coils of the snakes. And catching with both his hands the middle hood of the serpent-king, that highly powerful (Krishna) bent it down and set his foot upon the hitherto-unbended hood; and danced upon it victoriously. And the hood of the serpent was bruised by the treading of Krishna's feet, and wherever the snake attempted to raise his head, it was again trodden down.

In plain words: Once Balarama reminds him of his true nature, Krishna smiles and slips free of the snakes' coils. He grabs Kaliya's middle head, pushes it down, and dances on it. Each time the serpent tries to lift a head, Krishna stamps it down again.