I am not sure how to explain it. (I have never tried before).
I have been tuning motors since the days of carbs and many of them came sealed from the factory so all you could really mess with was timing and advance. An old trick was to swap how the vacuum advance worked. As from the factory as vacuum decreased mechanical advance increased. the result was a sort of static timing that only adjusted for the extremes of the "map" the trick we did was to connect the vacuum advance to secondary vacuum so as rpms increased and primary vacuum decreased secondary vacuum would increase and you would be putting both mechanical and vacuum advance in to the motor. This was good for allot of hp. You of course had to reset vacuum at idle when you make this change.
This lead to building of race car motors primarily for SCCA IT class and as you know the old rule set basically made messing with efi a no no ;-) so all you had left was to get the timing spot on and freeing up the intake and exhaust as best you can. I spent allot of time at the dyno perfecting this and the only way I can describe it is when you get it rite a motor will have a particular feel. A motor should feel crisp while still smooth. The instinct is to push timing till things go far. A motor with to much timing (but not knocking) feels more aggressive. Once you feel a properly times motor as compaired to one that has 1-2 deg of timing over optimal a couple times you will know what I mean. Maybe this is the black art part.
The other thing that dyno time taught me is that smooth and boring is usually fast (similar to driving style). Another really good tool is taking a knock sensor and wiring it to the microphone input on a PC or an old tap recorder and then listening to the motor while you accelerate. Knock is easily detected once you know what to listen for. Kind of like having popcorn in your motor. The reality is that if you are getting to that point on an NA motor with timing that is not extreme you have pushed things way to far. HOWEVER Boosted motors are a whole different ballgame. I am not even going to get in to that at the moment.
So getting back to my time line and how I got to this point. Now we have maps to work with. Cracking the mystery of the ECU and then the advent of MS have opened up allot more fine tuning and control but really what it does is very similar to the old vacuum / mechanical systems I started with just much more accurate and tunable as those were just vacuum and mechanical advance versus rpm and by tying the vacuum to the primary vacuum it in essence gave you the 2D map that we are all use to now when tuning with computers. The limiting thing is that you were basically stuck with a set advance curve over the rpm range versus load. The only way to change it was to change the weights in the mechanical advance or put a limiter on the vacuum causing delay or a maximum limit.
I have spent many hours learning about the timing maps due to me converting my 924s over to an cam fired EDIS based programmable ignition system. Looking at the maps either TPS versus RPM or MAP versus RPM (they really are similar) it becomes easy to see where you can put more timing and where you should pull it. I was fascinated and actually glad to see that these maps reflected what the combination of mechanical and vacuum advance were doing.
Now to complicate things when tuning you also have to be aware of the cams and where there performance range is as well as the overall performance window of the particular motor you are working on and obviously what a motor is going to be used for.
This is a really an overly simplified explanation. Trying to explain this is allot harder than I thought.
