According to Warren's [WFC] description posted on OzGem
Nick-1. Variable Reluctor - this is the standard rb gemini setup and is also seen in a lot of 70s and 80s GM and fords. it revolves around the fact that you have small magnets on a distributor shaft (One per cylinder) and they are equally spaced around the shaft (one per 90degrees in a 4 cylinder) you then have a coil sensor mounted inside the distributor that as the dizzy shaft spins these magnets will pass by the coil and induce a voltage in to the coil.
This will make a sine wave (in theroy) that will change in both frequency and amplitude as the rpm increases. The benifits of this is that the setup is completly passive inside the dizzy (requires no power source to operate).
this signal then will come out of the dizzy (2 wires, positive and negative from our coil inside the dizzy) and joins up to a 4 pin HEI ignition module where the other 2 pins join to the coil to trigger it.