I think if soundboard is perpendicular to floor, falling down weight is useless and it is actually a bad idea. It is more about arm counterbalance from shoulder to release thumb pressure. Some people overdo it, which is also not good and put too much stress over the fingers that must hold the heavy arm brute force. We also must be careful with restricting right arm that starts holding the guitar from slipping when too much left arm pressure is done.
In over thirty years of teaching, I’ve never mentioned arm weight as applied to the guitarist’s left hand. (Or, for that matter, the right hand.) And nothing I’ve read about it convinces me that I should. It’s a concept borrowed from piano pedagogy—for which it’s useful—and foisted onto an instrument for which it’s simply not apt.
If you’re holding the guitar correctly, the weight of your left arm pulls your hand away from the fingerboard, not into it. I understand that more perceptive teachers who advocate arm weight don’t mean it literally. Rather, it’s a metaphor to alert students to excessively gripping the neck. And by the way, many teachers who advocate arm weight do take it literally, even though it doesn’t work as they claim. (The objections raised by Luis begin to explain why.)
For teachers who view it as a useful metaphor, I would argue that there are better things to say to a student. For example, here’s a suggestion I’ve posted before:
Fret a note with your left hand finger (any finger, fret, or string will do) and repeatedly play the note with your right hand. As you continue playing the note, gradually let up the pressure with your left hand finger until the note begins to buzz. Stop letting up the pressure, but continue playing the note, with the buzzing. Now, as you continue playing, slowly increase your left hand pressure until the buzzing stops. Exactly at this point, stop increasing the pressure. You’ve now found exactly how much left hand pressure you need to get a clean note. Any extra pressure past the point where the buzzing stopped is excess pressure.
Notice how this exercise is perfectly tailored to whoever is doing it, regardless of physical differences between players. This exercise even tailors itself to different guitars or strings. Any person playing any guitar on any kind of strings will automatically get an accurate feel for how much pressure to use when fretting a note.
What I like about this exercise is that, for it to work, you don’t have to be sensitive to excess tension. And that’s the crux of the matter. To be useful, an excercise like this must alert you to excess tension even if you’re oblivious to it. In the exercise I’ve just described, the buzzing—something so obvious that anyone will notice it—instantly alerts you to insufficient pressure. And at the instant the buzzing ceases as you increase pressure, no additional pressure is needed. Even a player oblivious to excess tension can understand the folly of pressing harder after the buzzing stops.
(I make no claim that I invented this idea.)
To me, this little exercise is far more useful for alerting students to excess tension. Someone who’s taught this (or something like it) doesn’t need to hear anything about arm weight. Indeed, the arm weight idea is a roundabout way of sensitizing students to excess tension. Why bother with roundabout when something more immediately direct is at hand?
South Euclid, OH