Light sensor so if you are trying to activate by yourself/too close and no one on other side, it growls at you!
There are 3 of these, each getting more complicated for the person who activates.

1st one is just around the back side, second is both at same time [front/back] so hold hands, third is you have to slip your hand (or head) into darkness and hold as it powers up.

What are the ART images that you get to see? Perhaps you see a fade in camera like image of a girl in that white/depth mode moving, but barely visible, but only sometimes. The rest of the time you see infinity ocean waves in first, rain slipping down glass in second, fog/smoke in third.