Dean, detected out-of-phase surround ambience material which is entirely in the left front channel is sent to the left side surround only. That material which is also present to a smaller extent in the right front channel is sent mostly to the left back surround so that it images in the left rear. That ambient material which is equally present in the left and right front channels is sent equally to the left and right back surrounds so that it images in the back center. Similar analysis starting with the right front channel, of course. The concept is to theoretically get a transition of sound arrival directions more closely matching the recording venue.
