I have an Echo 1 tracer unit purchased from Jeff at Capital Arms. PM him and he can give you the information that you're looking for...
They are fun as hell and the strobe effect of the bb's flying thru it is insane! Getting shot at it is awesome too; the "muzzle flash" completes the whole night game experience.
The unit itself uses a light gate to activate the strobe much like a camera flash for each bb passed thru it.
I leave my tracer on my M4 as it has an off/on switch so there's no need to find it when you need it.