The Duck Tail offers what T-Rex calls its classic delay pedal sound, plus a very tasty tape delay emulation, but it gets its name from the ducking delay mode that keeps the delay in the background for a less cluttered sound.
Sounds
In Duck mode, you control the amount of delay you get via picking dynamics - a light attack gives you a full delay but you get less delay if you pick harder, while a sensitivity knob sets it up to suit your playing. This is beautifully implemented and lets you 'play' the delays, with the repeats clearly coming in during quiet passages.