What did Meg have to do to break the spell of the dark thing?

Suddenly, Meg realizes that IT is lying to her and that the one thing that IT doesn’t have is love. She begins to think in her mind about how she could love this thing, and she realizes she doesn’t have the strength to love IT. But she does have the strength to love Charles Wallace and so she starts repeating in her mind how much she loves him. This turns into her screaming out her love for the child and Charles Wallace begins to break out of the spell he's under. He runs towards Meg and they embrace.