This was a ritual developed by the "People in the Mirror."
It is called the Reflecting Mirror of Friends.
The purpose is to use the mirror as a medium, reflecting everything from the mirror on the other side; and to convey words to the holder of the mirror on the other end... to communicate with distant friends.
It also allows the ceremony holder to block the view in one direction, if they wish.
What the ritual requires is a "Radiant Mirror Surface" made of inorganic material, clean enough.
The left eye Elle gave to Annan was a gem of pure purity—it was also a mirror surface.
When entering the nightmare, Elle had returned to her most perfect state, that is, at the moment when her left eye had not yet left her... In other words, Elle's left eye existed simultaneously in her own eye and in Annan's eye.
This is undoubtedly a "predestined" pair of mirrors.
If Elle opens her eyes, everything reflected by the gem in her left eye will be reflected in Annan's eyes.
