If Mix Start is not set, the next track will be mixed at the Mix point of the playing track. For instance, if playing track has Mix point of 5 seconds, the next track will start playing 5 seconds before the end. That is, tracks will overlay for 5 seconds.
If Mix Start is set, say it's set to 2 seconds (from the start), the overlap in this case will be 7 seconds (instead of 5). Meaning, next track will be mixed in earlier.
This is useful when a track has quiet beginning, so you can mix it earlier, for smoother transition between tracks.