In Pepe Romero's book La Guitarra I cannot find any discussion of knuckles over strings, but the following paragraph is interesting:
Pepe Romero: One of the most important things to understand before discussing the right hand position and the free stroke is where the stroke actually originates. The finger should be made to move freely as a result of the impulse it is given by the forearm and the palm of the hand. The forearm is the place where the thought of the sound to be produced is transformed into energy. Then, the consciousness must be place into the palm, and from there the signal is sent to the finger tip, which acts as a sensitive receiver. It is at the finger tip that the energy to set the string in vibration is given direction.
Huh? Does anyone think they understand what he is saying here? It seems like he is saying to use the weight of the forearm to displace the string, but then he talks about the palm and looses me. In a later quote he seems to contradict himself:
Pepe Romero: In all playing, the motions of plucking the strings should be all in the fingers and not in the palm or the arm
He just said the stroke originates in the forearm and the palm. This is very contradictory and unclear.
An eyewitness will often only see what he already believes to be true.