it's basically a tap in between the preamp/EQ board output and the predriver stage board inputs.
the amp goes...
record player input --> pre-amp --> CD input (if equipped) --> EQ BOARD (if equipped)--> out through jumper plug and back in --> predriver boards --> transistors -->speakers
there is an pre-amp accessory board addon for adding external background music (part #60792702) that utilizes the port to bypass the input and inject your own signal (radio tv etc.). the jumper is there to feed the audio from the CD/record unit back into the amp for driving the output stage boards.