The pulse is (and I don't remember which comes first) a postive and negative going wave, and they of course "take time." That is, they are slightly one ahead of the other, as the reluctor tip moves past the pickup. The box triggers on ONE of them. If you reverse the wiring, this offsets the pulse in time, and the spark is triggered where the rotor "isn't." This has become known as "rotor phasing" and is much more important now with various EFI and electronic retard/ advance systems.
