So if a competitor site is launching, shut down your site for the day and redirect all traffic to their site in an attempt to flood their server? Interesting tactic but I don't think it's gona work somehow.
You would have to know your competition is going to be launched with a bare-bones setup, or at least that they have a considerably lower estimate of their launch users than you do. At that point, make sure your users know about the competition, and go down right when it launches. No guarantee it will work, and if they manage to weather the beating from increased users you will probably lose quite a few, but there is the potential...