TheAim SmartyCam RPM Bridge is used whenever the car has got no ECU (i.e. classic cars).In this case, the RPM data can be obtained with connection to the coil or reading a square wave signal generated by the ignition system.
RPM Bridge is an interface designed to allow RPM overlay on SmartyCam GP HD videos when the car has got no engine control unit (ECU) this can happen with classic cars or particular car categories.