I play the washboard in sessions some of the time (wind instruments more often). Some of you will have heard me do it: I'm a lot more precise than any other washboard player I know of, trying to get the same rhythmic effects as a dance band snare drummer. I use the thing rather selectively, and one reason for inserting it is to provide a metronome-like function - some pubs can be so noisy from ambient babble that it's quite hard for the players to hear each other. So I pick a player who I can hear clearly and who think has got the timing right, and reinforce their beat. There is no mistaking the kind of accentuation I'm putting in (whack on the wood with the thumb on the downbeat, rather complex fills on the metal with the fingers on the offbeats - basically like banjo frailing), and this instantly pulls everyone together. Often the effect will continue if I fade out, and I often will if it seems like a percussionless but still well synchronized texture will sound best at that point.