The Distributorless Coil Simulator simulates the original coil trigger signal thus allowing the ECU to properly trigger both the tach and fuel injection with the MSD DIS Ignition installed. In most cases, the Simulator plugs directly into the MSD's Harness. In some instances when you install a DIS-4 Plus Ignition, the factory ECU may not be able to distinguish when the coil fires. This is the signal that is also responsible for firing the injectors (or the tachometer), which may cause a no-run situation.