bx_solo is a tool that Brainworx designed to get you started with, and used to, the way they approach the M/S technique - be it for recording-, mixing- or mastering purposes. The "heart" of their M/S tools are the Solo Buttons that allow you to actually listen to all components of a stereo mix / stereo signal individually:
L(left channel), R(ight channel), M(ono sum) & S(tereo difference signal) - all phase corrected (when needed) and mono-ed out on both speakers of your system. This is helpful when working in M/S modes and to check your signals / mixes for errors, noises, clicks, whatever...
bx_solo also contains a stepped M/S stereo width control, so you can get an impression about what you can actually do with it.