Dolby Digital EX is actually a descendent of Pro Logic - there is a mono rear surround signal specifically mixed for the track, but it is not a discrete channel in the Dolby Digital bitstream (there wasn't room in the AC-3 spec for a sixth full-range channel). Instead, it is matrixed into the side surround channels in very much the same manner as Pro Logic encoded stereo signals from the days of VHS carried center and surround signals mixed into the left and right channels.

Just to toss in a bit of history, there were many early Dolby EX discs (Phantom Menace comes to mind, but there were numerous others) released without any mention of EX on the packaging or the disc. They also contained no "flag" identifying them as EX tracks, but if the user manually enabled EX decoding they'd get the extra channel. Discs eventually began using the EX flag, and Dolby also began modifying the spec for that flag and how it was supposed to force a receiver or processor to behave. Today, if the disc is mastered with the EX flag engaged (and it is still not always done - the latest Star Wars DVD's still don't include the flag on their EX-encoded tracks), the receiver is supposed to force Dolby EX decoding no matter what. It is intended to override everything else, including user preference (many of us prefer to use Pro Logic IIx to get separate left and right rear surround signals using IIx's superior matrix decoding scheme) and quantity of speakers in the system.
I wrote that last night thinking one thing, and need to dig back into DTS to be sure that I had it right. First, we have to remember that "true" DTS ES (sometimes called "DTS ES Discrete") contains a sixth full-range audio channel in the digital bitstream, unlike the way that Dolby EX works. Thinking about it after a little more rest (although not as much as I'd have liked), there exists some mechanism in the DTS standard to allow for retrieval of the sixth discrete channel when using systems that only off DTS decoding (such as receivers made prior to the introduction of the DTS ES extension). My suspicion in this case, though, is that the 970 is using the ES decoding to retrieve that data directly and them allowing the DSP engine to steer that data to the side surrounds.

There was once also a "DTS ES Matrix" mode offered on many receivers and processors that produced a mono surround back signal from regular DTS 5.1 sources (basically, a direct parallel of Dolby EX used with regular Dolby 5.1 sources). A couple years ago, DTS changed the way they identify that mode - it always used the NEO:6 matrix decoding scheme even though it was called "ES Matrix," but now they are referring to it as "DTS+NEO:6" and ES is only used with Discrete ES tracks. I don't know that all manufacturers have done this, but Outlaw's current offerings handle it this way.