its actually quit simple...
each sword has a different ability, so far the one we've seen forces Kyouraku and his opponent to play a game, in this case they say a color, and depending on the amount of that color you are wearing, the attack strength/damage changes. Its actually quite a interesting zanpakutou ability because you have to follow the rules that the zanpakutou sets.
Like when Kyouraku said "grey", Starks arm was cut, but because grey is a small part on Kyouraku, the damage was small. Likewise when Stark said "white", Kyouraku was damaged heavily because stark wears a lot a white. and when Kyouraku said black at the end of the chapter, Stark was HEAVILY damaged, maybe even killed because Kyouraku was wearing almost all black
as for the other sword of the set, it seems that who ever steps in the shadows is injured. I'm not totally sure about what its full abilities are because it seems too vague when Kyouraku explained it
how can you say the AUTHOR messed up on the explanation of HIS manga? now i'm not saying that you are stupid. What i'm saying is that Kubo explained it how he saw fit, and its also possible that there is something off with the translation because the Japanese language is extremely complex.