Renowned hand surgeon, who I've been in regular communication with, clarified for me, just last evening, the following crucial fact:
[Added to this recent revelation]:
When an individual contracts "only" the tip joint of a finger and allows the 2 larger joints to "do what they will", the middle joint contracts, but not passively. It performs work during this contraction, but said work is involuntarily.
[Here's the new information]:
Dr. now tells me that the large/ main knuckle joint extends under these same circumstances, but that this extension is not passive; involuntary work is being performed there too, just as is the case with the middle joint.
All movement at the middle and large/ main knuckle joints is involuntary when we intentionally activate only our tip joints and allow the two larger joints to "move as they will".
This explains why we see John William's largest finger segments extending, away from the palm, when he plays Recuerdos, for example.
Adding requisite pressure to the string via work at the right bicep and pectoral muscles, together with invoking the "earliness" parameter, intentionally activating the tip joint as early as is possible in order to bring the finger to the string solely via the intentional work at the tip joint [thus preventing the need for *intentional active work to be performed at either the middle or large/ main knuckle joint, at the exclusion of the tip joint] brings it all together.
Just to be very clear: work is in fact being performed at all 3 joints, but the only intentional work is being performed at the tip joint and this intentional work must begin early enough, in order to ensure that it is the tip joint's intentional contraction the causes the finger to be brought to the string, while the two larger joints perform their active roles *involuntarily*.
This hand surgeon will definitely be included in my coming book:
Classical Guitar at Your Fingertips: a Complete Manual on the correct and most Natural Way to Play.
To be honest, none of this would be happening without my surgeon; he's done more for the classical guitar than he could possibly imagine (at least not at this point)!