I saw this on the net today and I think it’s a great idea.
Personally I use Morse code, But this limits me in two ways.
1. I need a electronic medium to transmit over great distances.
2. The person receiving the message needs to know Morse code.
With his you could easily use a light or any noise maker.
Here is the Wikipedia page on it.
Here is a link to it over at Survival Tek http://survivaltek.com/?p=4617
Or a video by trailkeeper on youtube