In "A Wrinkle in Time," how is Charles Wallace finally freed?

It is Meg, Charles Wallace's older sister, who finally frees him from his mental slavery to IT. She does it by exercising the only power that IT does not have--the power of love.

She begins by kything to him:

"Charles, Charles, I love you. My baby brother who always takes care of me. Come back to me Charles Wallace, come away from IT, come back, come home."

Finally, she is able to look at him and cry out loud, "I love you, Charles! I love you!"

Then, after a "whirl of darkness," both Meg and Charles Wallace are transported safely home.