Tbg, it really is one channel out of phase (actually inverted polarity) with the other on the recording and then the two channels are in phase. If you reverse the wires on just one of your speakers the image will be centered when he says the signal is out phase and diffuse when he says they are in phase.
Your question is the tricky part. Once they are both the same polarity (image centered when he says they are in phase) the idea is to listen to the part when he says they are in phase, then reverse both speaker leads to invert the polarity and try it again. The bass should sound more natural one way or the other. As Rives pointed out, some people are more sensitive to this than others. Or perhaps some know what to listen for and others don't. I've never tried it, but I will later today to see if I can hear the difference.
Your question is the tricky part. Once they are both the same polarity (image centered when he says they are in phase) the idea is to listen to the part when he says they are in phase, then reverse both speaker leads to invert the polarity and try it again. The bass should sound more natural one way or the other. As Rives pointed out, some people are more sensitive to this than others. Or perhaps some know what to listen for and others don't. I've never tried it, but I will later today to see if I can hear the difference.