So what is M/S compression? Well, M/S is center and then the stuff that's on the outside of the stereo signal. So why would you use M/S with compression? Well, you might want to get the middle pumping, but leave the outside alone. Or you might want the opposite of that, where you've got the outside moving, but you want to keep a really nice tight center. That's the way that I would normally use it. Because the stuff on the side is frilly nice stuff that is just there to give the track a bit of movement. But really most of the stuff's happening in the center. And you really want a keep that nice and tight.
Now you can also, as a bonus tip, split it up into a multi-bank compressor and you can get the bottom just working on its own. So you get a really nice tight low end, or you can just get the top going. So its doing some nice little sort of fairy dust across the top. So check it out M/S compression.