Duckdelay is a way to lower the delay level while you play…so that the delay repeats are louder while you pause and softer when you play. Delay will not interfere with your playing.
Xnob = Ratio Sets the ducking ratio or the degree to which the delay is attenuated. How much the delay level is lowered.
Depth =Threshold: Sets the ducking threshold – the audio amplitude – at which ducking kicks in (-36 dB to -66 dB). In simpler words it sets the level of your playing at which the delay is lowered.
Speed = ReleaseTime: Sets the release time from 500 to 10 msec. With the release time set to short values, the delay will kick in quickly when you stop playing. With the release time set to longer values, the delay will stay ducked for a while. Longer release times are useful when you’re playing a riff and don’t want the delay to kick in between notes.
The ducking delay is VERY dynamic! It really depends on your playing dynamics, so a bit of parameters tweaking is required to fit anybody's playing techniques and levels.
hope this helps