Thanks Jeff, I took your advice and I do think I have a better understanding. I guess now I just need time and experience tasting lots of good and bad espresso before I become a true master.
Over extracted = bitter astringent flavors
Under extracted = earthy flavors
I never found tamping to have any significant effect on shot time, or extraction for that matter.
What do you mean by tampering not affecting the shot time and extraction? For me the relation is obvious. Of course, there are other factors. Grinding being the most important (?).
If your tamper is very consistent, of course, you will find no difference. But try the same espresso machine and grinding with different baristas (or with begginer baristas)...
The water pressure of your espresso machine at 9 bar is equal to around 250kg of pressure. which means you can probably do a hand stand on top of your puck and it would still be far lower than the pressure of the pump. A little experiment for you to try is to tamp as hard as you can and pull a shot and next don't tamp at all just place the tamper on top of the coffee with no pressure and pull a shot. I've done this many times and the difference is insignificant (maybe 2sec difference which is probably more to do with headroom in the basket than actual tamp pressure).
Any difference between Baristas is likely to be more about dose differences or uneven tamping which creates channeling rather than tamp pressure.
If you look at the different factors that influence actual levels of extraction in coffee the number one factor is brew ratio (the amount of water to coffee), next comes Grind/shot time (I put them together because if you dose to the specific requirement of your basket (i.e 22g in VST22) grind changes will change extraction time. Grind changes by it self has only a very small, somewhat insignificant effect on extraction, i.e. 22.5g vs 22g in VST22 extracted to the same brew ratio in the same time will extract almost the same despite different grind setting). I don't include dose in this list because it is only a significant factor if you are extremely overdosing or underdosing your basket (lets say 2g over or under the designated dose), in that case your extraction could only drop. then there is temperature and pressure but they have a rather complex and confusing influences on extraction and flavor which are not completely linear.