You have an audio track, and there’s a typical noise gate.
Then there’s another audio signal that wants the original track to duck, like DJs do with compressors.
But normal ducking doesn’t allow the 1st track to resume control.
In particular, if I have a guitar picking out notes (Track 1) and I have a digital delay of that (track 2), then I want the delayed signal to completely cut off the tail on the note on track 1, until track 1 exceeds a threshold when the next note is played. The idea is to hear only one track at a time, so that notes that are not in harmony don’t interfere to create dissonance.
The special gate would work like a normal gate, except that it would close if the (A) the input falls below the normal threshold level, or (B) the side-chain signal exceeds a (possibly different) side-chain threshold. It would open whenever the normal input exceeds the normal threshold. Applied to both tracks, you’d get mutual exclusion.
I’ve been able to create this effect in a round-about way. I just think it would be an interesting thing to easily do.