So if you look at the schematic above, that ties the jacks together when no plug is inserted.
In practical terms, it’s the return jack that does the switching, labeled as output in my diagram.
Think of it this way:
1.) When there is no plug inserted in the Return, the circuit preceding the loop shorts to the output of the subsequent circuit.
2.) When a plug is inserted in the Return jack, the switching contact lifts and breaks this pass through connection. The send takes the preceding circuit output to whatever is plugged into the loop and the return accepts the output of the device plugged into the loop and passes it onto the main circuit.