The mods are the five switches in the upper left corner of the SCK-30 (a.k.a. SC-300), and two buttons in the right that create static noise.
All the switches modify only the rythms, they isolate each component of the rythms; bass, snares, and the accents. Two of the switches create a feedback, one of them only affects the bass creating a more artificial sound for it. The other one creates an overall feedback making it sound higher.
Now listen to the tracks caught by the fisherman and fisher tribe and you will be able to notice how is done using the mods and some delay provided by the korg ES-1 .
Soon I'll be posting tracks made with the Casiotone MT35, and a video showing what can be done with the mods on that one.