Originally Posted by crossg

Dan knows what he is talking about.
From my Onkyo manual:

5.1-channel source + Dolby PLIIx Movie
These modes use the Dolby Pro Logic IIx Movie mode
to expand 5.1-channel sources for 7.1-channel playback.

Another:
Neural THX 5.1/7.1
Neural-THX Surround employs psychoacoustic frequency
domain processing, which allows delivery of a
more detailed sound stage, with superior channel separation
and localization of audio elements. The Neural THX
5.1 and Neural THX 7.1 modes can expand any 2-channel
stereo source for 5.1- or 7.1-channel playback,
respectively. Use them with CD, radio, cassette, TV,
VHS, DVD, and other 2-channel stereo sources, including
video games. Neural-THX Surround can also be
used by broadcasters to encode and transmit surroundsound
content over a stereo signal, which listeners can
enjoy as either surround sound or normal stereo. XM
Satellite Radio, for example, using Neural-THX Surround
on select channels, which the AV receiver/AV
amplifier can expand from 5.1 channels to 7.1 channels.

There are more listening mode that do matrix from 5.1 to 7.1.