I've struggled to describe it because I've only heard a handful of systems do it and that is my fault for not being able to communicate it.
Some systems create a flat plane for the music.
Some systems create a curved plane.
Some create depth which gives it a flat plane with music that happens behind 'main stage' - a box of sound if you will.
Some create a semicircle in front of the listener.
Some create a full circle around the listener.
Some create a full circle around the listener and fill in that space with images.
The last bit is what I am trying to describe - a projection of sound not only behind the speakers, but instead utilize the entire listening space as a soundstage.
The full circle is awesome, but getting the area within that circle to as a soundstage for music is unique and creates the true atmosphere of a performance.
I've discussed these before but here are the two songs that really showcase what I am talking about.
Deadmau5 - 4ware. This song can be quite repetitive on systems that don't fill in the space. There are pulses of sound and you hear them but don't experience them. On a sound with that atmospheric circle, these pulses actually start behind the speaker and end up coming at you in waves at about three or four levels until they reach the listener or extend slightly past the listener. The interesting thing is that they take up distinct space in that circle and 'approach' the listener with each pulse. They don't hug the perimeter of the soundstage.
Tool - Chocolate Chip Trip. About 1:35 into the song a drum starts in the front right corner of the room and will do a full 720* around the listener. In most systems, including many components that I sell, this won't happen. It will circle around to directly left of the listener, collapse back to the left speaker, pan to the right speaker, then jump back out directly right of the listener. This is an example of the semicircle. While you get a sound that 'reaches out', it doesn't actually reach out completely. It cannot create the full circle and cannot fill in the space within that circle.
One thing I don't know fully yet is if you create that full circle, does the space automatically fill in? I don't think I've experienced one without the other yet.
Right now I don't have my reference DAC and my current DAC is a bit of a compromise. It creates the circle with atmopshere, but it is 'squished' like an oval in front of the listener, never getting parallel or behind. It does fill in the space that it is capable of reaching out to though. It's a weird situation but even using that, I'm able to showcase what exactly it is that I am talking about and they can grasp what happens when the reference DAC is in place.