EDIT: oops....spent so long thinking about how to say this simply that it doesn't need saying any longer
Side is Left-Right.
So basically you're phase inverting one channel and nulling the 2 channels. The mono material is identical in left and right so it completely nulls to silence. (or as RJ pointed out if panned mono lowered in level by the null)
The result is the stereo only component of your audio - The mopnitor of this is in place so not summed to mono.