Many movie soundtracks have a mono surround track - especially those recorded prior to 1993. If you are sitting closer to one surround speaker than the other, the sound collapses to the one side instead of accurately immersing you in the onscreen action.
THX Adaptive Decorrelation subtly adjusts one surround channel’s time and phase relationship with respect to the other surround channel. With THX Adaptive Decorrelation the listening position is expanded, providing you with the same spacious audio experience as in a movie theater.