Mono to stereo circuit only if one cable is plugged in
It will likely use a mono jack socket with a transfer switch like this: -

Then inter-wire NC and NO contacts like this (blue and purple): -

So, if the LEFT jack is inserted it will naturally connect its tip to the amplifier and also connect the tip to its NO contact. That NO contact connects to the right NC contact (blue wire) which connects to the right tip circuit when the right jack plug is not inserted: -

The purple wire is for when the right jack is inserted.
This should produce a left and right balanced mono signal if either left or right jacks are plugged in. When both are plugged in, you get normal left and right stereo and no cross-connection between the channels.