I Solemnly Swear That I Am Up to No Good: Fred and George?

Um, Ron and Sally-Anne and I were working with my dad's book, and we were trying this one spell that's a kind of protective spell. It's really cool because the protection is invisible, and it worked, because she didn't feel anything at all when we threw a whole bunch of hexes at her.

Except when Ron tried Petrificus Totalus. That did something, we don't know what, like, uh, the protection sort of became visible. And we don't know how to, um turn it off. It looks really strange. Any ideas? Finite Incantatum isn't working.