Even if your phone is off, you make the effort to show them it's on and that you're recording the incident and calling the police. I bet anything these kids would feel more stress thinking that someone not only took a picture of the car and license plate, but also recorded the entire incident so that they pretty much wouldn't be able to defend themselves should it go to court.
I've faked up enough of those myself. When I was in Chicago, I'd fake that I was calling the police and taking pictures of the taxis... they'd go from belligerent road ragers to scared little boys in seconds, whipping away in their taxis before I could get a picture of their taxi numbers.
Koffee