good question. I’ll give it a shot. by inverting the phase of one of the channels and summing both to mono, you’re essentially subtracting, or nullifying (through phase cancelation) the identical information from both channels - which is what the mono signal is. So, all that’s left over is side information. The second gainer inverting the phase of one channel returns it back to stereo info instead of the mono signal the stereo expander device produces