An equilibrium outcome of a mechanism can be reproduced by asking for types and then sending the messages prescribed by the original equilibrium strategies. Truthful reports are then a Bayesian Nash equilibrium: a profitable false report would induce a profitable original deviation. This reduces optimization over indirect mechanisms to direct revelation mechanisms with Bayesian incentive compatibility, under the same information and participation assumptions.
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The Revelation Principle is a concept in mechanism design, a field of economics and game theory. It states that for any mechanism or system designed to achieve a certain outcome or allocate resources, it is possible to design a direct mechanism (or mechanism with straightforward reporting) that achieves the same outcome when participants report their private information truthfully.