This method is named after the famous Frank Baum book (and more famous movie) with the same name. (If you’re not familiar with the book or the movie: in it, the great Wizard of Oz is impersonated by an ordinary human hiding behind a curtain.) The “wizard” (the designer or someone else familiar with the design) controls the user’s screen remotely from another room. None of the user’s clicks or taps really do anything. Rather, when the user clicks, the “wizard” decides what should come next, then serves up that page to the user’s screen. The “wizard” may even create the page on-the-fly and serve it up. Users don’t know what produces the response. In fact, they are usually told very little beyond that the system is “unfinished and slow.”
Wizard of Oz testing is particularly useful for testing AI-based systems before you have implemented the artificial intelligence. The human who controls the computer can simulate the AI responses based on natural intelligence.