Espresso seems something of an esoteric craft. There is a ubiquity to coffee, and no such mystery to that substance. People know, by in large, what it is, and how to prepare it. There are many more advanced techniques, of course, and plenty of idiosyncrasies to indulge if you'd like. But electric drip coffee makers are the stuff of just about every kitchen, often requiring no more than the flip of a switch to operate.
Espresso is not like that, I'm reminded frequently. It's not that there are pointed questions often -- though when there are, it's something of a treat -- so much as there are confused looks, abstract inquiries, and awkward misstatements.
This is forgivable ignorance, as only the smallest of minorities will ever use an espresso machine, and thus have cause to know exactly what it does, and how it does it. This is not to imply that there is a reputable, widely accepted barista handbook out there, detailing the proper way to tamp, tap, twist, and pull.
Truthfully, even that statement is too specific. There is a substantial school of thought that drifts decidedly in the "less is more" direction. For these baristas (I wonder, do they also run barefoot?), only the tamp is necessary, and even that should be subtle. Tapping only serves to loosen grounds; twisting is best left to wedding receptions and drunken relatives.
Still, it's not hard to find a good barista who does all of those things, and believes their espresso would suffer without them. And there are talented minimalists as well, pulling good shots with little more than a 20lb tamp.
Esoteric? Yes. Impermeable? If one is looking for a definitive truth, then yes. In that regard, espresso might be compared to basketball, or (the aforementioned) running, in that the result determines the validity of the means used to acquire it.
But even then, there is uncertainty. A good shot, in basketball, is one that goes in; it's not so easy to define a good shot of espresso. Volume varies, as does flavor, consistency, color, and everything else you can thing of.
Clearly, this is not a guide to espresso, nor is it meant to make it any less murky a subject. I'm not sure that there is an authority on the stuff, but I'm absolutely certain that if there is, it's not me. As for my technique, I currently use this approach: One hard tamp, two light taps, another softer tamp.