I'm doing research on alleged testimonies and evidences of the spirit realm. In particular, I've been lately reviewing testimonies and live recordings of exorcisms, some of which I find particularly impressive. I say impressive because in my opinion the exorcisms look quite convincing, and I lack the expertise in psychology and neuroscience to provide a convincing explanation other than assuming that everything is staged (which I wouldn't be able to prove either).
Common patterns are that the allegedly possessed person seems to switch personality (as though the alleged entity's personality is taking over), sometimes the exorcist and the alleged entity establish conversation, sometimes the person starts to "throw up" the alleged entity out of their body right before full deliverance is achieved.
What is the psychological explanation for exorcisms?