I've been going back to these things over the last year and I've found a lot more tricks. Middling sized ones with deep ridges give you the most options. Can't do such complex rhythms as (ten-finger) washboard but the sound is quite different. I can easily pull off far more complex patterns than the guy in that St Chartier clip.

They have a problem you don't get with any other instrument. I was playing in a gastropub and came back from the loo to find the barman clearing them away with the dishes - they use identical shells to serve things on and he assumed mine belonged to the pub. I suppose bagpipers used to have the same issue in taverns where wine came in leather bags.