Consider it as a contrast to the first kanji, ken, 顕. Ken refers to things that are manifest, that appear and are visible, tangible. In contrast to this, yuu 幽 refers to things that are not visible, tangible, or made manifest in this world, though they do exist. Action is tangible, manifest. Thought or intention is not.
So in this interpretation, yuu
幽 refers to the world of the mind, hence Ellis' reference to the subconscious. Would this also have been Ueshiba's interpretation?
I am remembering a French class I took a VERY long time ago in which the professor was analysing Racine's (I believe it was Racine's) reworking of classical Greek drama in which the gods, who had been external forces, were now treated as internal mental states. Is this similar to the differences in the interpretation of yuu