Oops, there'll be lots of links but I can't remember one right now.
OK, let's try:
Impedance balanced means that for each of the two signal-lines the output-impedance is the same as 'presented' to the next piece of gear,
and it'll be in combination with only one line actually having signal on it.
The other line is then just simply a passive network connected to ground and that network is chosen as to mimic the output-impedance of the 'active' like as well as possible.
The thought behind al this is that you get the good things (disturbance-rejection) of balanced lines because of the identical sensitivity of both conductors for disturbances, so that's about impednaces of those lines.
Whether there are signals on those lines of opposite polarity OR just on one line doesn't matter ! The following sym.-input simply subtracts the nastyness on both lines and your disturbances are reduced.
You do get some improvement from both lines carrying signal but that's a bonus coming from twice the signal. But the essential part lies in the identical susceptibility, which is determined by the impedances. So make these as equal as can be.
So there's no signal of the opposite polarity present on that second line,
something to keep in mind when you go to an unbalanced device
and want to use signal-swapping a few lines to get the other polarity.
Here's an example:
http://www.jensentransformers.com/as/as004.pdf
(C3 & R4 are mimicing C4 & R5.
The output impedance of the 'active part' is about zero, so little or nothing has to be added to C3 & R4)
Hope it's more clear now - it's a simple concept but hope I've written it down not too hasty.
More good info:
http://www.jensentransformers.com/apps_wp.html
Bye,
Peter