This plugin allows you to convert from and to MS and adjust the level of the M and S signals. MS stands for Mid/Side, and is traditionaly used as a recording technique with the aid of a MS microphone.
M stands for Mid or Mono, and is nothing more than the sum of the left and right channel. S stands for Side and is the difference channel, or left - right.
After LR-MS processing the left channel becomes L+R (M) and the right channel becomes L-R (S). You now can for instance process these channels individualy.
This is a mastering trick. For instance, compress only a voice signal in the middle, or enhance the stereo width of a cymbal by using eq on the S signal only.
Afterwards you should encode back to stereo (MS - LR). You can also use the plugin to convert a MS microphone signal to stereo using the MS - LR option.
In LR-LR mode it can widden or shrink a stereo image. In this mode the stereo signal is converted to MS and the faders can than be used to adjust the level of the M and S signals independantly.
After this the signal is converted back to stereo. Try it on a stereo channel and move the M fader down a little. You are now removing some Mid signal and you'll notice the stereo image widens.