So, what's happening with their arms?

Unknown. Hyung's morphology is as mysterious as 4chan's existence. :/

If the symmetrical docking is because of there supposedly "mysterious" arms. It's not needed. The woman in the white is grabbing onto the other womans arms. And the woman in the black. The one whos hands are being grabbed has her hands on the other womans hips. There's nothing weird in this image at all.

From the wiki entry: "A term to describe the embrace of two female bodies so that the breasts interlock, hence “symmetrical docking”." If you don't understand what a tag means, it's a good idea to look there.

Symmetrical docking has nothing to do with the arms or weirdness at all.

The right arm of the girl in white has no wrist, her arm suddenly ends in fingers

Magsgame said:
The right arm of the girl in white has no wrist, her arm suddenly ends in fingers

Once you see it, oh my goodness you can't unsee it.