Are you sure its not MAIN IN and Preamp OUT?
Typically, that jumper links the pre-amp (output) section of the receiver to the power amp (input) section.
So, if you want to drive a separate power amp, you would connect an outboard power amp to the pre-amp OUT connections.
If you wanted, you could also use the receiver's power amp section separately by connecting an outboard pre-amp to the power amp IN connections.
Typically, that jumper links the pre-amp (output) section of the receiver to the power amp (input) section.
So, if you want to drive a separate power amp, you would connect an outboard power amp to the pre-amp OUT connections.
If you wanted, you could also use the receiver's power amp section separately by connecting an outboard pre-amp to the power amp IN connections.