It is the one command that people will listen to. It is an amazing technique that you can use whenever you want.
The sentence you say to somebody when you spot something weird or unknown that is on them. Almost always, they will immedietely become still also known as the "Don't Move!" response. The person will almost always think that it is some sort of insect on them. Particularly a spider.
Sometimes people use this to elicit the "Don't Move" response from friends just to piss them off.
Is it a spider? Oh my god I can feel it in my hair!!