Since the amperage load during cranking drops the battery voltage somewhat, the bypass wire allows the coil to get approximately the same voltage while cranking as it does during normal running through the resistor.
There is a few pitfalls that can get you here. The original system was setup to use ballast resistor, a ignition resistor bypass circuit (attached to your starter switch) and a non resistor coil (usually marked on the coil as to whether it needs an external resistor or not).
Allot of backyard mechanics are under the impression if you run a coil with an internal resistor you can eliminate the ballast resistor. As Jerry pointed out, the system was designed to supply full voltage to the coil while starting then switch over to reduced voltage while running. This will allow full starting voltage and protect your points from burning up to quickly.
First check to see if the coil does not have a built in resistor like the photo below. You want one that requires the external resister then make sure either resistance wire or a ballast resistor (like the photo below) is installed inline to the wire supplying voltage to the coil. Finally attach that new wire from the starter switch to the coil. Now you’ll have two wires attached to the positive terminal on the coil.
When I bought my truck it was wired without the resistor and without a bypass starting circuit. I found out thru this forum that allot of these old trucks have been running without one installed for years.
