It's a built in grounding switch that in conjunction with a relay turns on (grounds) at a set rpm and then turns off at another set rpm. This essentialy creates a "window" of operation for an electrical device such as N2o. Ever see a manifold explosion at the strip? When a driveshaft, axle or tranny lets loose the engine over revs and hits the rev limiter. If the nitrous is activated, the hood scoop will launch. lol! With a window switch you can deactivate the nitrous automatically at a pre set rpm below the rpm at which the rpm limiter is activated. This will save you a lot of work, time and money if something goes wrong. No one likes intake manifold shrapnel, drivers and spectators alike!
They are not waterproof, although waterproof boxes are availible from both MSD and Mallory. This adds considerably to the cost as they are potted in epoxy, and I wonder how this effects their ability to dissapate heat. I always mount my boxes in cab as there is less exposure the elements, heat and vibration and I can see the digital (LED) readout in the bright sun as it is mounted under a dark dash. lol! Always use rubber grommets when you pass wires though the firewall too.
Did I mention to "Never weld on a car with an aftermarket ignition installed without first unplugging the harness"? OK, just reminding everyone. ;)
