Why when i sidechain my vocals to a reverb fx channel for ducking, when i go to choose the input which i want to activate the compressor i have placed on this channel to duck the reverb, it gives me options for send/output on the sidechain drop down menu, when in all the videos i see there is only one check box not labeled at all. im confused about this. mine says Send on one side and Output on the other, when i choose send it gets louder and then output i know the ducking is working but am confused as to why there would be a send option? what is the purpose of these choices?