If I remember right, it's the use of thinner-gauged strings to transpose the lower three. The EAD moves up an octave to kinda compete with the GBE. Each time you strum through a chord, you something more like a melody than steps up a scale.
It sounds really interesting too. When I play the 'Nashville' tuning I hear a lot of recorded guitar parts from soft rock tunes. I think it is meant to be good for recorded music as it clears lower frequencies without the need for EQ.
