It's talking about violating the principle. The word principle almost always means a conditional claim on LSAT. So must be false would have us thinking conditional, and principle would also have us thinking conditional. Sure enough, the word unless is there. Unless is a very common conditional trigger word, and one of the easiest ways to always know you're setting it up correctly is to learn that unless is the same as if not. The if part of a rule goes on the left. And if not is just saying, we'll put the negated version of this on the left.