Shorting the inputs means taking a jumper wire with alligator clips and hooking one end to the center pin and the other end to the outside ring. One can take an old RCA cable, cut it and tie the 2 wires of the cut end together and plug in the connector to the amplifier. They make special plugs for this function as well of which I have.
Cutting an old RCA cable and doing like I said serves the same purpose for testing. The plugs are put on RCA inputs of a Preamplifier, especially phono stages, to prevent noise. Common on professional equipment as well.