GUE specific communication ?!

[eelnoraa]

But it's easy to signal vertically with the back of the hand facing the other diver the same way as when the hand is horizontal.
Although I don't quite understand this back/palm when the obvious difference is vertical against horizontal... What am I missing?

The difference between 1-5 and 6-10 is definitely the facing of the palm, NOT orientation of the fingers. I have worked with 3 GUE instructors, there is no confusion about that amoust them.

"But it's easy to signal vertically with the back of the hand facing the other diver the same way as when the hand is horizontal."

Here you are thinking about divers facing each other, and probably at the same depth. How about when divers are in single file like in smaller passage? How about diving are side by side like scootering?