a prototype that only works by having someone behind-the-scenes who is pulling the levers and flipping the switches. The wizard of oz technique in user testing has a user interacting with an interface without knowing that the responses are being generated by a human, not a computer. This allows testing of some difficult interface concepts before a system is fully working.
For example, in a search system, a user may type in a query, and an expert behind the scenes rewrites the query in a formal syntax or hand-selects search results. This allows you to test theories in query formulation and filtering of results. Similarly, the wizard of oz technique is popular in testing natural language interfaces where, for instance, the choice of what syntax to support in the system is driven by what syntax users actually use during the tests.