The next line after “If you didn’t tell her at all what the spell was supposed to do, it would stop working.” says
If she knew in very vague terms what the spell was supposed to do, or she was only partially wrong, then the spell would work as originally described in the book, not the way she’d been told it should.
Knowing that the spell is ‘For enemies’ apparently counts as knowing in very vague terms what it will do.
The next line after “If you didn’t tell her at all what the spell was supposed to do, it would stop working.” says
Knowing that the spell is ‘For enemies’ apparently counts as knowing in very vague terms what it will do.