You have to sum the original and the inverted polarity copy back together, if they null you'll see no waveform amplitude on a waveform graph and if you are listening by ear, you'll hear nothing, when both play summed. It's important as Brad mentioned to make sure that they are sample accurate, because doing this in real time, latency can cause phasing, which will not null and give you a bad read when indeed they may null and there are no true differences when sample for sample on the timeline.