Essentially, it slightly reduces the stereo separation and does a few magic tricks to reduce the possibility of FM multipath.
FM multipath is when the signal bounces off more than one surface, and each signal hits your radio at a slightly different time, which creates unstable reception. The multipath mitigation tool in Breakaway One attempts to minimize this. It's one of those features that, if you don't know what it is, you don't need it.
Josh