All I'm saying is that for a full tone, both the middle and knuckle joints should flex together (though at various degrees) during the pluck.
I agree that it's possible to get a less full tone by flexing middle and knuckle together, but I'm not convinced that it is a rule. I'm also fairly convinced from the video evidence that world class players will do an immediate knuckle lift in technically demanding situations like tremolo, fast free strokes, etc. For fast free strokes, staying in front of the string is needed to be ready to pluck--immediate knuckle lift is what keeps the finger in front of the string. Sometimes it looks like the lift happens at the release side of the nail, but the knuckle is still rebounding off the string and the knuckle never follows through into the palm for any of the fast stuff. (slow passages are a different story)
I can play off the extreme left side of the nail, slicing through the string, and at an angle that's more on the parallel side of 45 degrees and get a very warm, full and loud sound. When I'm in this position, it is impossible to get the nail under the string to pull up on it and get a slapping sound, or even a thin sound. So, I can pull straight up if I want and that pulling up has zero adverse effects on tone. Also if I pronate my forearm a little bit, use a fairly standard 45 deg angle, make sure the contact is nail and flesh, and slice, I can still get a warm, loud sound while lifting the knuckle during the pluck.
To avoid futile debates that may result from different perceptions of the visual evidence I suggest using relative instead of absolute valuation. Thus my claim: the more uniform the pluck, the fuller the tone. Conversely: the earlier the knuckle lift, the weaker the tone.
Thanks for the suggestion, I'll think about doing that. I agree that often we are talking about the same thing or 99% the same thing and are arguing a small difference. I don't think there is always just a two-sided model that fits, most times there are many differences of opinion. I did explain in my first post that everything I say is just conjecture and mostly observations of what I see in the videos. Others are capable of posting their own videos or re-posting the video and explaining what they see--that's actually what I hope will happen since I'm doing all the heavy lifting so far. People can draw their own conclusions from the video evidence.
An eyewitness will often only see what he already believes to be true.