How does Geraldine caste her evil spell on Christabel ?

Christabel undresses herself and lies down to sleep. Yet Christabel is so worried about Geraldine that she cannot sleep and instead sits up in bed and watches Geraldine praying. While praying, Geraldine suddenly shudders and undoes the belt around her waist, causing half of her body to be exposed. At first, Geraldine does not move or say anything when her dress falls. Then she opens her eyes and sees Christabel looking at her. Geraldine hurriedly and shamefully lies down in the bed next to Christabel and takes Christabel in her arms. Geraldine says that a spell is working in her and that this spell is in control of Christabel. Geraldine says that Christabel still has the power to fight against this spell because of her attempt to save Geraldine. You can check more out at the link below:

http://www.gradesaver.com/coleridges-poems/study-guide/summary-christabel-part-i-1797-part-ii-1800-the-conclusion-to-part-ii-1801