Sometimes English speakers call sanzas “thumb pianos” because they share the same principles as a piano keyboard: they’re digital, in that you get one and only one note from a key on a sanza. About all you can do with a sanza is play repeating rhythmic cycles. It’s not an instrument for expressive melody, it’s an instrument for melodic rhythm. You play it with your thumbs, which Africans can do with great rhythmic independence between hands, generating a complex hypnotic groove to accompany song and simulate dance. To make it louder, they set it inside a large calabash to resonate the sounds.
At this point we have the prototype for the modern Latin dance-band piano player. The guajeo, or repeating rhythmic cell that the piano plays in Cuban dance music, treat the piano as rhythmic percussion in very much the same way the sanza has functioned for—probably—thousands of years.