Prefader aux send   

Signal is sent via prefade aux send fader to aux send master independently of level of input fader.  (Hence the term "prefader")  Useful when the aux send and master output need to be completely independent.  Some possible applications include sending the prefader aux send to on stage monitors or to headphones in the studio or to a process where the wet (processed) and dry (original) signal need not be related. Older mixers and broadcast mixers may call aux sends "foldback" or monitor sends (usually meaning pre-fader) and echo send.
see foldback post fader