Two wires provide power to run the electronics in the sensor. When conditions are met the sensor output is grounded, and when the conditions cease the signal line is released. What the ignition does with that signal is not related at all, and depends on the design of the ignition unit. The circuit probably begins charging the cap as soon as possible after firing in order to handle as much RPM as possible. The signal line should actually be held high (+5V) and the sensor is sinks current when on to drop the line to (~0V, logic).