Pins on M50 coil? What does what? And what kind of signal over each?

[Bill R.]

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It is my understanding that Pin "1" is ground, Pin "15" is +12v, and pin "4a" is the trigger wire?

If this is true, while the car is on, does the coil always have GND and +12v? What kind of signal goes across the 4a pin? I cannot find it in any bentley manual, etc.