"Advanced students get their hands on books where they do teach it, for a detailed analysis," Malfoy said. He was right, Hermione did know the spell but she was hesitant to say it. She contemplated what would happen if she told him, but also thought about what would happen if she didn't. If Malfoy failed to deliver what he'd promised to Voldemort, then Voldemort could kill him and it would be on Hermione’s hands, not to mention what Malfoy’s cold heart would do to her. Unwillingly, Hermione spoke the incantation.