When I was learning I went through what you're going through: I had HEARD about something called tap tuning and was mystified and eager to learn about it. I presumed that it held THE secret for easily-replicable world-class results. I actually thought that the alchemy of guitarmaking was somehow locked inside this arcane act of wizardry called "tuning" the top, that it was something that only the select and most sensitive few knew about, and that they weren't going to tell me, so I would somehow have to learn it myself.
Builders, alas, often do things on guitars which they really don't know WHY their doing it, but they do it because they were taught to do what the teacher did, and they're afraid if they stopped doing it, the nice sounds they usually get will go away. So they keep doing it. Then, when you ask them why they do it, they are all too happy to MAKE UP vague, fanciful, jargon-laden accounts which will leave an impression that they know exactly why they're doing it in minute detail, and by implication demonstrate that they can somehow successfully manipulate all these invisible sonic phenomena on the guitar with ease. Luthiers usually won't stop you from believing that they are wizards. This is most painfully obvious in the realm of "tap tones."
Yes, you can tap a top to derive some rudimentary information about its anatomy. What you hear does give you some feedback clues which is useful and helpful. The "tuning" part is what is so misleading to beginners. At some point (most often toward the end of the process) many builders attempt to make some final changes in the anatomy of the soundbox, which the builder believes (I've selected this word carefully:) believes is exercising some control over the final results. Both, skillful experts and deluded fools, equally, scrape here and there, tap, press, reach inside, remove a little on the back braces, on the top braces, and then at some point say THERE. It's just right.
Then there are other builders, masterful experts and deluded fools alike that DON'T. They believe that they're making all the crucial decisions in the INITIAL stages of the construction: materials selection, materials dimensioning, design: that is all they need to achieve the desired results.
My good friend and colleague, Alan Chapman is confident that he has come to appreciate the difference of fine changes in bracing heights, soundboard thickness, patterns, and has come to BELIEVE that he can goose the sound quality of a certain note, or smooth out, or sweeten, or balance, or adjust the performance of distinct notes and distinct regions of the fingerboard by subtle changes in specific areas of the soundboard. With all respect to my dear friend, Alan, I believe that this is delusional. The soundboard is responsible for the production of a very limited part of the entire guitar sound spectrum. Yet his results are terrific. So I have to swallow a good portion of my skepticism when it comes to Alan's guitars. Yet, I know of other makers whos results are equally phenomenal that do NONE of that.
That is why I say now, like I said fifteen years ago in my book, that "tuning" is something that can't be taught. But its also true that luthiers evolve idiosyncratic actions that they believe lead their guitars towards better "tone." This means that over time, each maker devises a series of actions which conform to a refined mental model that they've derived of how the guitar functions. This happens as a result of making, thinking, worrying, fussing, cursing, tossing and turning, eating and sleeping guitars for years and years. The mental model becomes more and more refined, and more and more PERSONAL. Jimmy D'Aquisto lectured that he made oval soundholes in his acoustic guitars, so the sound would "squirt further" like water from a similarly-constricted hose nozzle. The audience snickered when he said that. But nobody in the audience was getting $22,000 per guitar like he was. Snicker at that.
