Dave Firestine calls it the "universal chord." It's what you get when you lay the fingers of your left hand loosely across the strings of your mandolin to damp them. It's no chord at all, and it works in any key. You turn the mando into a rhythm instrument.
Works with guitars, too.
What's it good for?
Learning rhythm for one thing. It makes you aware of what you can do rhythmically with your right hand, without distracting you with having to form chords. You can play the universal chord as soon as you can pick up the instrument, and play around with all the different things you can do with your right hand.
Texture's another. If you're backing someone -- playing mando duets for example -- or are in the middle of a trio or quartet, you can play the universal chord once through the tune as a way to give it a different texture.
Backing up a singer is a third. Loud's the norm for jams. If someone's trying to sing a verse, you need to back off so he can be heard. The universal chord doesn't step all over his melody line.
Jumping in to tunes you don't know is a fourth. If your chord-hearing skills are still weak, and you can't just listen to a tune and tell where and what the chord changes are, the universal chord lets you still jump into the jam and contribute.
Understanding how much you need to press down on the strings to damp them's number five. When you haven't done this much, it's a good way to learn it, quickly. You can also hear that, even though you're damping the strings, damping them in different places gives different sounds. As with kettledrums, "percussion" doesn't mean "tuneless 'thunk.' "
Can we make it an even half-dozen? Sure. You learn to play loudly enough. Far too many beginners think they need to be dainty in their actions. After all, someone might hear them and ask them to leave, or something.
Remember back when you were self-conscious dancing because you didn't know what you were doing and you were sure everyone was watching you look dumb? Then, one day, you realized that everyone else was busy worrying about how they looked and weren't looking at you at all? And how, after that, you had a lot more fun and danced a lot better?
Same thing. The universal chord lets you open up and play forcefully without worrying about playing the wrong notes.
