MSD RPM Activated Switch, shipping is not included These RPM Activated Switches will perform a variety of different functions from turning on a bulb or solenoid to activating an MSD Timing Computer at a desired rpm. The RPM Activated Switch, PN 8950, 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.