Four dials under the frame are controlling the speed of four squarewaves made by the Hexinverter. These squarewaves are switching "on" and "off" the four adress pins of the demultiplexer. Depending on the adress, the voltage on the common IN/OUT is distributed to one of the moving coil mechanisms. The voltage is forcing the needle of the moving coil mechanism to move and hit its physical restriction. And then it falls back. This is causing a nice percussiv sound.