The "5 Second Stadium" stopwatch only counts to five, making it at first glance suitable only for timing jet car races or Brownlee's mood swings. But what it's really timing is your brain's internal clock; stop the watch too early or too late and various character voices will scream at you for your poor sense of timing.
It's probably a fairly useless skill to develop — and one obviated by a real stopwatch! — but it's kind of a charming toy all the same. It's Japan-only for the moment.
5 Second Stadium teaches you to count to five [Trends in Japan]