Why was Beca's riff-off considered invalid?

Question

A couple of days ago I watched this new musical comedy, Pitch Perfect. Here's one question that puzzles me: Why was Beca's riff-off considered invalid? What "it" did they need to match and how? I've watched this particular scene, like, 50 times and I still have no clue. Maybe there's something in the book that they forgot to include in the movie?

Answer 1

Jesse's song used the word "it" in its lyrics. Beca's rap riffed off this word. However, her song began with the word "it's" and was therefore deemed invalid on a technicality.