i just read a couple of articles on how a CD ingition works. it's entirely possible that the small red wire is pulling voltage in on/off cycles. what happens is the power charges a capacitor in the MSD. when that capacitor is 'full' it no longer needs voltage. as you rev the car, the cycles happen so fast that the guage can no longer keep up (visually)and the gauge looks to steady out. without seeing a wiring diagram of the insides of the unit i can't say for sure. i'm also not clear why the box requires 2 input voltages (heavy and small red wires).