I am (obviously) not an analog guy so bear with me on this....It seems to me that whether the switch is NO or NC it has to be connected to some source of power to generate a transition to whatever is counting the rev's. I'd imagine it's a normally open switch that momentarily closes to complete some circuit and this generates a clock pulse to a counter or something along those lines. But wouldn't that quickly drain the battery if the switch was left in a closed state?