I wanted to elaborate a bit on the Ida Presti Technique stuff I posted before. The big thing about guitar is that we want to make a good sound. Every classical guitarist in the world wants to sound good and have a nice, full tone.
Part of what Artzt talks about is the idea that the string vibrates perpendicular to the soundboard rather than parallel with it. This isn’t a new idea. Nor is it something that can only be accomplished playing off the right side of the nail. Left side, more “traditional” players can easily get the same effect.
A rest stroke makes a big sound due to this desirable motion of the string. Think about it: we push the string down, then snap off coming to rest against the next string. We force our finger to move through the string, causing motion perpendicular to the sound board. This can be accomplished with free stroke as well. It’s just a matter of moving all the joints on the right hand in the same direction. Drop something on the floor. Now pick it up. Notice how you wrap your fingers around it into a fist? That’s how your hand should generally move when playing a free stroke.
Think about pulling your fingers through the strings then having them follow through back into your palm. Another way to think of it is using all flexors (the muscles that pull your fingers in) to make the free stroke, followed by relaxing outwards to return to the starting position. This should instantly make a louder, more full sound from which a great tone can be cultivated.
The big thing to remember is that our hands should move very much the same when we play guitar as when doing other activities. This is a good starting point to work from. Obviously more advanced technique requires a bit more cultivation.