In the middle of the movie, the main character (the piano teacher), breaks one of her students' hands by placing a bunch of broken glass in the pocket of her jacket, destroying her musical ambitions forever.
That she did this out of her jealousy of the girl's contact with her love interest, Walter, is obvious, but it's also a multifaceted action. From IMDB:
"She destroys the musical prospects of an insecure but talented girl, Anna Schober, driven by her jealousy of the girl's contact with Walter -- and also, perhaps, by her fears that Anna's life will mirror her own."
I also believed at the time that she was disappointed by her student's failure, and so she broke her hand to destroy her musical ambitions. I saw this as part of her icy, dictatorial, perfectionist personality.
Could someone explain the motivations of this action in sufficient detail?