A few different pedal voices, overall pedal volume, and a pedal celeste effect (a strange stereophonic detuning and phasing effect). And more lower-manual accompaniament instruments to go along with the automatic rhythm unit. And that red button is to control the sound of the faked (human) vocal ensemble sound for the upper-manual (it adds a lower octave), it's a rather strange, and not too nice, warbly–reedy sounding voice.