So, this is something I discovered with my PSR-E433, but I would guess that it applies to the entire PSR-E400 range, but if someone finds otherwise, please let me know.
It has to do with using the sustain pedal with organ sounds. I found this out while coming up with an arrangement for the Genesis song, "Abacab." If you're familiar with the song, you'll know that, after a quick guitar lick at the beginning, it goes to 3 sustained organ chords -- Csus4, Fm7, then G7sus4 -- before going to the main song. While these organ chords are sustaining, there is a raspy synth lead playing. So, what I wanted to do is to quickly hit the organ chord on my PSR-E433 with the sustain pedal pressed down, then jump to my Roland synth for the raspy synth sound. I was hoping that, by pressing the sustain pedal, that the organ chords would keep playing indefinitely since a typical organ envelope does not decay as long as you hold down the keys (I was hoping that pressing the sustain pedal would be equivalent to holding down the keys). Well, it didn't work that way.
It turned out that, when I hit an organ chord with the sustain pedal pressed, and then let go of the keys, the organ chord faded out within 1-2 seconds. But then, when I was experimenting with another organ sound I have in a different registration, I found that those chords actually DID ring on indefinitely after I let go of the keys, as long as I had the sustain pedal pressed.
So, what's the difference? The difference is that the first organ sound I used was made up of panel voices (main voice: Rock Organ, dual voice: Cool Organ), while the second one -- the one that did ring on indefinitely -- was made up of XGLite organ voices. I have not tested this with all panel and XGLite voices, but it does appear that the XGLite organ voices will ring on indefinitely when you let go of the keys (if the sustain pedal is pressed down). And it appears that the panel voices will fade out, instead.
Of course, the panel voices are generally higher quality, but I was able to come up with a very similar sound by changing the main voice from the panel voice Rock Organ to the XGLite Rock Organ (and beefing it up with the volume, chorus, and filter settings) while keeping the dual voice as I had it before, the Cool Organ. Yes, the Cool Organ part of the sound will still fade out, but with the XGLite Rock Organ still ringing on, it's not that noticeable -- especially when I'm playing the synth lead over the organ sound.
Hope this helps with anyone's sound designs using organ voices!