The GMX decoder uses a mode hierarchy to select the most appropriate post processing mode. For example, Dolby Digital 5.1 soundtracks will be autodetected and decoded as 5.1.
For Dolby Digital 2.0 signals, the most appropriate mode is assumed to be Pro Logic II Cinema mode. Pressing the surround button in this situation allows to adjust the mode selected by the hierarchy. The need to do so after skipping tracks or pausing your DVD player depends on the operation of the DVD player. In many cases the Dolby Digital signal is lost, and must be reacquired. Once the signal is reacquired, the system runs through the mode hierarchy again, applying Pro Logic II Movie mode.