I have been contemplating the r****d unit you have.....
and, yes.. it can be connected across the coil..
Its actually quite simple. (the good ideas always are LOL - I was doodling a design last night and bingo.. I saw the light.. hee hee
This is how it must be working..
the coil is normally connected to a positive supply and ground - thereby charging the coil.
when a spark is demanded, the -ve terminal is floated - ie - disconnected from ground. The field collapse (Back emf) of the coil that results is the force that fires the plug.
Sooooooooo.. your little unit must be doing the following..
By connecting in parallel with the coil, the unit can keep the -ve terminal grounded AFTER the amp has effectively made the terminal open circuit. hence, delaying, or retarding the ignition.
the rpm is simple to calculate from the pusles applied.
So, ignore what I said before ... and yes.. it can go across the coil.
obviously it will only work when activated ie - WOT switch or whatever - when the gas is on.
very simple, very clever..
hope that sorts it out..