Cranking an engine causes a heavy load on a battery which can cause the voltage to drop. Ignition systems needed to be designed so they can fire on this reduced voltage, but when the engine is running normal operating voltage is restored, which is then too high for the ignition system. The ballast resistor helps the engine to fire more easily by being bypassed during cranking, and then lowering the voltage when it is inserted into the circuit after the engine has started to minimize wear on ignition components.