Thanks for taking the time to explain in further detail...I do appreciate it. However, I think you left out the key point of my observation that I'd invite you to consider. Please realize though that these are just my own casual observations. They're certainly not scientifically formulated by any means and could be partially or wholly inaccurate, and may not account for any differences in machine models or operation.
AssafL wrote:Air is a spring.
You're absolutely right. You could even say that once the basket is locked into the group head there is more air trapped behind the puck than within the puck itself. However, I'm just commenting that I find that springiness is inversely proportional to the flow rate. The lower the flow rate, the more the springiness. And the higher the flow rate, the less the springiness. And that the air is still trapped somewhere in the pressure chamber either way.
AssafL wrote:And this as long as air is the dominating factor - it doesn't matter if you pressurize the puck and then Let it dissipate and then PI or if you first PI and then pressurize the puck.
There is a third option here that I mentioned in my last post. It's not a very tasty one, but it was the reason behind my thought process. And that is pressurizing the puck and not
letting it dissapate. By locking it up and keeping the high pressure, eventually the entire puck will saturate and begin to flow out with the tiniest trickle and pretty much stay that way throughout the shot. In this scenario the the puck is fully wetted and yet the flow rate will remain hindered.
AssafL wrote:This springiness goes away once fingering flows find the bottom of the basket.
My own experience has been that in the lock up, constant high pressure scenario, after the puck is eventually fully saturated there is a window in which it will remain springy before the flow rate increases. Probably easier to demonstrate this on a lever machine. I'd call it a 'window' of a period because after the puck is saturated you will still be able to bounce the lever (springy) several times, although with each bounce the density of the fully saturated puck is slowly loosened up (much in the same way a clogged sink gets loosened via plunger).
AssafL wrote:In regards to tamping pressure and undoing it - even 30lbs is so inconsequential compared to the 8-9 bar that it is unlikely.
Here's another part of what's led me to my thoughts about that. If I grind really super fine with no tamping whatsoever (just a fluffy, very full basket) I can still manage to get a 25-30 sec pull and a great tasting shot with crema, albeit with a little different mouthfeel. But if I were to tamp that same super fine grind I would lock it up for sure. So I've always found tamping to play a pretty big role, especially in regards to grind size.
Anyway, I found the whole thread very interesting. And my own experiences are definitely in agreement with RapidCoffee's conclusions about preinfusion and mouthfeel.