When to use THX Mode?

My McIntosh HT preamp has a THX Mode, however, I find myself rarely using it because most movies aren't in THX. The manual can be vague, stating that the mode is "for movies originally created for a movie theater." Anyone have any insights as to when to best use this setting, if not only for THX films? Is it really for THX films only? Thanks, you guys really seem to know a lot about these things. When my DVD comes back from the shop, I'm going to try to figure out once and for all whether I should be selecting DTS, Bitstream or PCM signals to send to the preamp (and I'm sure I just mangled the terms, thich shows how little I know) Thanks --