call and response
LEDs can be used to detect light, here two LEDs are trying to get synchronised, if they detect bright light when they are off a number of times, then they 'know' they are synchronised, and the blue LEDs light up to show this.
Randomskk goes further to have a bit of information transferred over the link
Using processing.org as a quick video capture software.
The arduino patch is from www.flickr.com/photos/randomskk/
and google ledcomms for the sourcecode and his youtube video