Some high end tape recorders allow you to actually listen to the recorded matter while recording it. There will be a delay and the quality is generally not as high as the regular full size "playback" head.
On my Pioneer amp, when I hit Tape Monitor is like switching to the tape channel, press again to return to previous channel. It does that so I can hear the recorded tape. If I had switched my amp to listen.... well there would be a big feedback because I have my amp "tape out" going to the recorder, it would start to record itself. Using Tape Monitoring with selector set to CD or the source continues to send the CD sound to the tape, but the speakers begin to play the currently recording tape. Nifty.
Therefore your desk should send the master channel to tape send just ahead of where it blends the monitor into master for your speakers, otherwise massive squeels of feedback should occur.