What does a B on a note stem in guitar music mean?

Question

I found B's on note stems in sheet music of the 3rd movement of the Libra Sonatine, "Fuoco", by Roland Dyens, which is for guitar: What does that B on a note stem mean in this guitar music? Those notes with B's are not played entirely cleanly, and part of me suspects that the B has to do with barre, but I still cannot figure out what the B is entirely for (unlike Z's on note stems earlier in the music, which are for mordents).

The sheet music scrolls by with the music in the following video, and notes with B's are played starting at 13:49 in

(the video should start at 13:49).

Even after taking a good hard look at Roland Dyens playing "Fuoco" in

(he starts playing notes with B's at 3:14, and the video should start there), I remain confused - he didn't even seem to play the notes with B's in a consistent fashion, especially not the ones in the last measure of the piece.

For what it's worth, my Google searches for what this symbol means have produced only irrelevant results, and I couldn't find any guide on what those B's mean earlier in the sheet music.

(When I made an Liszt's-Paganini-Etudes-style enhanced transcription of "Fuoco" for piano, I ended up treating the notes with B's as if the B's weren't there. To this day, I am not completely satisfied with how I treated those notes.)

Answer 1

Not sure what it is, but it sounds like a snap pizzicato, alternatively called Bartók pizzicato (that could be that B -- this technique was first used on violin by Bartók, so it's sometimes called by his name).

It is performed like this: instead of plucking normally, you put your finger (it's best done with thumb) under the string (between the string and the guitar body), pull the string upwards and release. The string gives a tone as usual, but there is an added "percussive" sound as the string hits the frets and rebounds.

(Alternatively, you can do it with two fingers like John Williams in this performance of the Usher Waltz. I think you will be able to hear the effect more clearly in this video, too.)