For each note, there is a wind channel, and the slits visible in this picture go into these channels. There are pieces of wood with leather bottoms called pallets prevent or admit wind into the channels. The soundboards are upside down so that we can see the bottom. On the top are sliders, one for each set of pipes. By moving the slider, holes either line up with the pipes or stop the wind entering the pipes (hence “stops”)
Here we see the old rexene being removed in preparation for restoration.