At a recent symphony concert, on one particular piece the basses were switching back and forth a lot between arco and pizzicato. Each time they would make the switch, they would reach down and move something up or down, on one of their strings. The thing they were moving -- whatever it was -- was located between the bridge and tailpiece. They would move it up when they prepared to play pizz and then move it back down again when they prepared to play arco (or I may have it reversed, I can't remember which direction was which). Anyway, does anyone know what it was they were moving, or adjusting, on that lowest part of their string? Also, I don't know which string it was (my eyes aren't nearly that good).