The RPM-Activated Switch will perform a variety of different functions from turning on a bulb or solenoid to activating an MSD Timing Control at a desired rpm. It has two activation wires; one to ground a circuit and the other to open a circuit. Simply plug in an rpm module and wire the Switch to the circuit you want to activate. When the engine rpm reaches that amount, the circuit is activated and will remain on until the rpm falls below that amount. No rpm modules are supplied so they must be ordered separately.