It’s pretty straight-forward. A knob which changes how much a track is sent to its output while “cued”. If it wasn’t sent at all, you could do DJ stuff where you hear what you’re playing in headphones before it is sent out.
@Seph_Reed if you (or anyone else) still wants to propose this feature request, then the template needs to be added again. Otherwise we can discuss this idea as a #brainstorm topic.