All in black and white from page 83 of the PSR 3000 manual:
http://www2.yamaha.co.jp/manual/pdf/emi/english/port/PSR3000E1.pdf DSP/DSP VARIATION
With the digital effects built into the PSR-3000/1500, you can add
ambience and depth to your music in a variety of ways—such as adding
reverb that makes you sound like you are playing in a concert hall.
• The DSP switch is used to turn the DSP (Digital Signal Processor)
effect on or off for the currently selected keyboard part.
• The DSP Variation switch is used to change between variations of the
DSP effect. You could use this while you play, for example, to change
the rotating speed (slow/fast) of the rotary speaker effect.
Personally I've delayed buying a PSR 3000 BECAUSE THE DSP VARIATION SWITCH is MISSING! Damn.
Jerry