From the June 20, 2012 comic

I actually forgot to rant in the commentary about the faulty logic of Sarah's fantasy given that the watches are specifically designed to only work if you know full well what the watches are, what they do, how they work, what code to use, etc. Even if all the other safeguards failed, the whole "you think this X times before releasing to cast" is kind of a deal breaker were Tedd to actually give Nanase the wrong watch.