They modify the crank trigger signal between the trigger and the DME. Basically intercept and alter... Steve
some e36 are one solid unit and you cannot take them aparthttp://www.realoem.com/bmw/showparts.do?model=CA53&mospid=47490&btnr=12_0599&hg=12&fg=05
It electrically is the same. There are 4 coils in there (I do not know if they can physically separated though). The plug has 6 wires: +12, Ground, and 4 signal wires.