I think I agree with Ouroboros here - Hemisync seems to do some layering.

Actually, when reviewing DTZM and what was written about it in the forums there the author recommended to use a low-pass filter (cutting of all frequencies below a certain frequency) on the sound track (bowls and rain, etc) to avoid creating accidental monaural entrainment.

Imagine having a carrier frequency of 180Hz (the default) and another sound that on both ears is some Hz above it. You would, I guess, get binaural entrainment from the difference between the both ears, but also some monaural entrainment from the difference between base frequency and the other sound. Maybe this effect is actually used by Hemisync?

I'm thinking this is the case because I found this years ago:

http://www.mind-gate.com/OBE/sound/7117.html

This is in German, but what it does is this: For each Focus state it describes how many tracks you have to overlay, what the carrier frequency is and what the beat frequency.

Focus 3
59 [1.2] = r 59,6 + l 58,4
110 [1.3] = r 110,65 + l 109,35
288 [3.7] = r 289,15 + l 286,15

So, for Focus 3 you need three tracks: 59Hz carrier, 110Hz carrier and 288Hz carrier. Now you overlay them with beat frequencies of 1.2Hz, 1.3Hz and 3.7Hz respectively, resulting in the listed frequencies for each ear respectively.

I don't know why they do this (if they actually do this), but I've read before that people made tracks that overlaid different brainwaves - for example Gamma or Beta on relaxation sessions.

Oliver