he seems to be using the knuckle (in concert with each finger's other digits) in the reverse direction to pluck/pull the string away from the soundboard. Are you seeing that as well?
Yes, that is what I see and believe is going on. I resisted the idea that the knuckle could be moving away from the palm while the middle joint is moving towards the palm because so many classical guitarists will vehemently argue that doing so is wrong. (it contradicts the Aaron Shearer uniform direction of joint movement, and what most teachers and methods advocate). I was also resistant because I was taught from the very beginning it was wrong to have joints moving in opposite direction and so many great guitarists that I admire teach the uniform direction principal--Kanengeiser, Tennant, etc. But I can't ignore what I see in the videos and what I feel works best in my own hand. It surprises me after seeing all the videos I've posted that people can't see the joints are moving in opposite directions.
I'm not saying in any way that the knuckle is not contributing to the stroke. You can see that the knuckle brings the finger to the string and can put some force/energy into the string--displacing it a lot, a little, or barely at all. But after the knuckle brings the finger to the string, it's the middle knuckle that takes over the plucking and the knuckle bounces off the string in the opposite direction. In all the videos where they are playing fast, the knuckle joint does not follow through at all--all forward momentum of the knuckle is halted once the finger reaches and presses the string. The actual plucking movement (the initial sound or the loudest part of the waveform) comes at the instant the middle joint starts moving towards the palm and the knuckle is simultaneously moving away from the palm. What makes the pluck IS the middle joint moving towards the palm WITH the knuckle moving away from the palm. It's this instant that you would align with the metronome, not the knuckle joint flexion--which is the pre-pluck. That's not to say that one is more important than the other, but I think of the movement that's in sync with the metronome as the thing that generates the pluck. Given the first joint to move towards the palm is the knuckle, it can appear that it's the knuckle that generates the pluck, but it's not the case. If you try to pluck the string with the middle joint locked in position, and just moving the knuckle, all the knuckle can do is displace the string--it can't actually pluck, try it and see. You need to move the middle to get a pluck. This is true with both rest stroke and free stroke. Maybe there's some good reason to practice following through from the knuckle after the string for beginners or special cases, but from my point of view this is dangerous since the one thing that nobody is doing in the videos is following through from the knuckle joint. Sure they do it at slows tempos or when demonstrating, but never at fast tempos.
An eyewitness will often only see what he already believes to be true.