It's actually a mask with a concave face that looks exactly the same as the face on the outer side that you'd actually see if someone was wearing it. When it turns towards the inward side your mind (is supposed to) pretty much ignore you're looking into a mask and make it look like the face simply turned which is what you saw.

This happens because of the ways our minds process things, and without going too in-depth the reason this is an interesting test for schizophrenia patients is that they often don't see it as the face turning at all and think it looks obviously like a mask because their mind processes things differently.

Source: am a psychology major, spent the last semester learning about schizophrenia