In other words, if you connected nothing to the front of the bay, the mixer's direct outs would be connected to the tape deck's inputs automatically. This is also good for insert points. You can place the sends in the top row and returns in the bottom row. If you don't connect anything, the signal passes through the bay from the send to the return. Some effects require you to supply an identical signal to both channels for a stereo effect, and placing your effect unit inputs stackjed would allow you to mult into both channels with one cord.
Some people recommend to isolate all gear and lift the shields on one end of all balanced lines.
This eliminates all possible secondary ground paths, and should technically reduce or eliminate hums. I'd get a painful shock if I touched both amps. If the chassis were grounded together, you would have blown a fuse before possibly electricuting someone. I personally keep all equipment tied together for grounds. All power comes off of a single circuit.