Whilst I have never fitted one to my own car I was always under the impression that the ignition was cut momentarily to the engine allowing unburnt fuel from any / many cylinders into the exhaust which the spark plug then ignites, so whilst a live feed is required, I would assume again looking at the picture, this is controlled by the black box circuitry to the coil negating the need for a spilt circuit from one of the cylinder leads as this box is acting as an independent trigger / voltage supply for the spark.
What I am not clear on from the picture is how the ignition is cut / or not and if it just relies on an engine running richer on overrun and unburnt fuel being a result or enthusiastic revving?