This is a really interesting technique that seems to be really useful when trying to master bad mixes where you don't have access to the original mix or project. As ever this one was discovered on a weekly trawl of GearSlutz where I regularly seem to find new (to me) and interesting (to me) ideas and ways of working.
The way I use it is to use two copies of the stereo file in a DAW on separate tracks, then use MSED as an insert switched to "encode" with the "side" muted then a linear phase EQ then another inserted MSED instance switched to "decode" with "side" muted. I do the same on the copy of the stereo track with "mid" muted.
I tried this out on a really old (1998) live recording of a band I was in where we have an old tape recording from the desk in the venue. Like a lot of recordings from then (who knows what happens now, it costs so much to get a recording from the desk nowadays) the mix is pretty extreme - loads of bottom end on the bass but no mids, very trebly hard panned guitars, not a lot of drums (it was a loud drummer so not much needed to go through the PA in such a small venue) and a fairly decent vocal sound.
I am definitely not a mastering engineer but this is a really interesting method of splitting the signal into the mono only and stereo only elements and bringing the bottom end into line and balancing the stereo more than you might think possible - just by using EQ and panning on each of the two copies of the original stereo track. It's fascinating to use.
There's much more helpful info here: http://www.about-audio-mastering-software.com/mid-side-eq.html
If you've got dodgy old tapes this could help - the people I was in the aforementioned band with seem to have dug a lot up recently...