I'm really oversimplifying the explanation and I've tried for a while to wrap my brain around how the 5 pin ECU gets power through the dual ballast resistor, especially at "start".
The way I understand it, the 1.2 ohm side is in use all the time, feeding the ECU on start and run, and then the coil on run only, but the 5 ohm is used on 2 of the pins of the ECU during start and only 1 pin on run.
It would seem like it shouldn't start or run at all with the 5 ohm resistor side bad, yet I've had that failure (not start, fire on key release) more than once and seen it several times (phone call.. "Hey John, my car won't start...") and been told by pros that it is a common problem.
I'll even go as far as saying the ballast resistor failure rep that Mopars have can be traced to the 5 ohm side of a dual ballast resistor!
I was curious enough that I looked-up a wiring diagram. This is not the most nicely drawn diagram I saw but it is colour-coded and well-labeled, so helps with the explanation.
We know that the ignition switch was designed to only power one of the START or RUN circuits at a time, and the other circuit is open. The best explanation I've seen for the 5-ohm BR connected to ECU pin-3 is that it was to provide lower voltage for the digital electronics in the ECU, and they got rid of the need for it later because they redesigned the voltage regulator inside. ECU pin-1 is the main power pin, but if the pin-3 explanation is correct, it was power for only the analog electronics in the original 5-pin style ECU.
When the key is in RUN: The red circuit is powered from the battery. Both resistors of the dual BR are powered , and the ECU gets full voltage to pin-1. The coil gets reduced voltage through the 1.2-ohm resistor.
When the key is in START: The pink circuit is powered from the battery. The coil gets full power, not through the BR. Current flows in the opposite direction through the 1.5-ohm resistor. The ECU is powered through the 1.5-ohm BR. Assuming that the current draw of the ECU is small, this won't have a negative effect on its operation. (Since millions of cars were wired this way, it's safe to say that is the case.) The 5-ohm resistor is also powered this way, so the actual voltage seen at ECU pin-3 may be reduced during cranking.
If the 1.5-ohm resistor is failing such that it has high resistance, you may see the symptom of engine ignition during cranking but dies in RUN because the coil isn't getting enough power. If the resistor is completely broken, the engine shouldn't start at all, since either the ECU or the coil is unpowered at any time.
Possibly if the 5-ohm resistor (and possibly both resistors) is failing in a high-resistance condition, you may get the case where the engine won't fire in START because the combined resistance is too high so the voltage is too low going to pin-3, but will fire in RUN.