I don't get it.
From what I heard, it was conceived on the basis of what colors are commonly perceived to be "pure" (i.e.: untainted by other colors), meaning: red (+a), green (-a), yellow (+b) and blue (-b)...
However, when you pick some color along any of those axis (and therefore supposed to look "pure"), they don't look "pure" at all.
For example, the "pure" red looks slightly but clearly slanted towards magenta/fuchsia, while "pure" green looks more like green-blue.
Also, the colors are supposed to be distanced proportionally to how they are perceived to be different, but that doesn't seem to be the case: for example leaf-green and deep-blue are placed almost at the opposites, but leaf-green still looks more similar to deep-blue rather than blood-red even though there is less distance between them