The length of the blade is precise. Only by passing through the gate of the eyes can it reach the brain and kill. It's really very simple.
12 seconds sound clip from the Revenge of the Ninja movie soundboard.
You can hear this line at 00:33:20.272 in the Blu-ray version of the movie.
Quote context
[...]
- You're an expert on that stuff. Do it for me as a personal favor.
- What the hell do you call this damned thing?
- It's called a star shuriken. Ninja weapon.
- Ninja, is that one of those martial arts things?
- Couldn't an athlete, a professional, throw one of these things? Why not?
- Come on.
- It's impossible. I'm opening up a new gallery tomorrow. I have no time.
- Come on. I can't believe what you're doing to me.
- Tell them Jap friends of yours to lay off my family.
[...]