KORG PA4X 76 - Manual (Page 510)

506| Effects for the Sounds
When “[G]Envelope Select” is set to Input, the gate is controlled by the level
of signals that are the combination of the dry sound and the reverb sound.
When the signal level exceeds the threshold, the gate opens and the reverb
sound is output.
Normally, set “[G]Input Reverb Mix” to Dry (the gate is controlled only by the
dry sound). If you wish to extend the gate time, set the “[G]Input Reverb Mix”
value higher and adjust the “Threshold” value.