Taken me QUITE a while to find a suitable picture, mainly because YOU DIDN'T SHOW THE FEKKIN MODEL NUMBER!
Heh! Anyhoos, maybe you can see the principle? At some point in the circuitry there will be a "breakout" point that is nonetheless kept in circuit by the jack switch.
The standard operation is that plugging into 'send' gives a feed but preserves the signal thru' the rack unit. However, plug into 'return' and the link is broken in favour of an external device.
There will be other issues such as the operating levels of the S/R feeds and where in the circuit they are taken. Often these parameters can be adjusted.
Dave.