Thanks from me too. I can hear the scale rise and I can hear the low overtone (or is that an 'under'tone, or is the higher note really the overtone?). I can understand how, if one raised the pitch of the higher note and also increased the volume of the lower over/undertone VERY gradually, one might not notice it ended up in the lower octave.

I catch this with wind instrument (like flute or pennywhistle) players sometimes. They don't blow quite hard enough to get the note in the octave they want and they wind up sounding like they're playing the same note in two octaves at the same time. I suppose that would be pretty interesting of one could do it on purpose and consistently! (Or maybe people DO, and I just don't know about it.)