" Blanket permission for Harry to use his wand against Snape. Acknowledgment of one of Harry’s strengths, in a manner that is useful for the lesson. This is what it looks like when Snape actually teaches Harry magic. It’s a private lesson tailored to him because he needs the protection, delivered as sincerely as Lupin’s Patronus tutoring. "

― Lorrie Kim , Snape: A Definitive Reading