Is
this the lap counter that you're refering to?
I have seen that lap counter before, and always wondered specifically how it worked. The RS lap counter has a main unit as well as a "reflector" unit. The main unit has two IR sensors in it, and the reflector unit has two IR LEDs in it. The lap counter measures the time that it takes from when the IR light beam is broken between one set of the IR LEDs to the second set (hence measuring FPM).
Judging by the pics and the battery requirements of the lap counter in the link above, it appears that counter only has one unit, which is why I always wondered exactly how it worked.