As I said, there are instances of use for the back region, but when most people (even martial artists) use it in reference to movement they're talking about the entire pelvis and waist combined.
You can make up your own usage for it, but Japanese people won't understand it.
The kua is something quite different.
Strange as it was Japanese who taught us it and my Japanese friend who demonstrated it. But there again they were martial artists and in fact Aikidoka not Hiroshi the local shopkeeper.
Anyway, until you can use it as such and know where else in martial arts it is used and which ones compare to how I use it here then there's no more to say really.