Running two ignition systems gives you peace of mind during a long race, but how do you swap the coil wire to the back-up ignition? With these automatic coil selectors from MSD, that's how! They have two posts that connect to the voltage towers of the coils, while a third post connects to the distributor. When you switch to the back-up ignition, the coil selectors sense which coil is being fired and then transfer that coil's voltage to the distributor cap.