GDuckDly is a digital delay that allows the wet signal to be compressed by the dry signal.
Using high levels of feedback in a digital delay can create a busy sound. This can obscure the original, dry signal. In particular, it can make vocal tracks incomprehensible.
The compression in GDuckDly is configured so that the effect is turned down when the dry signal sounds. When the dry signal is quiet the effect is turned back up. This allows the dry signal to be heard over even extreme delay effects.