It's purely force of suggestion.
6 seconds sound clip from the The Exorcist movie soundboard.
You can hear this line at 01:10:04 in the Blu-ray version of the movie.
Quote context
[...]
- No. Why?
- Have you ever heard of exorcism?
- But... it, uh, has worked, in fact... although not for the reasons they think, of course.
- It's purely force of suggestion.
- You're telling me that I should take my daughter to a witch doctor. Is that it?
- Karl? Did you put this in Regan's bedroom?
- Karl, if you put this in Regan's bedroom, I want you to tell me. Now, did you?
[...]