I can't detail the procedure here, but the timing can not "advance" electronically, it can only retard. Think about it. There is no such thing as time travel. LOL. What happens is that the computer generates "delay gates" (pulses) which turn off the timing trigger coming in. This re tards the spark. As "more advance" (less retard) is needed the delay gate gets smaller
What you are going to get into here --and you have to do some searching and research--is what is called "rotor phasing" because in a normal distributor, the trigger/ spark event as compared to where the rotor is pointing---is determined by the position of breaker points or the trigger device. But with EFI the computer MOVES that because of the trigger delay. So the rotor position needs to be ?? physically timed so that the rotor is mid point on the cap contact with "about midpoint" in the timing curve---so that as timing is moved from "advance" to retard, the rotor is still in range of the contact.