In this slide deck, p15. it says "The revelation principle requires that the principal can fully commit to the terms of the contract. If this is not the case, an indirect mechanism, which allows for some commitment, may strictly outperform any direct revelation mechanism, which allows for no commitment."
It sounds contradictory, because the first sentence sounds like "For revelation mechanism to outperform indirect mechanism, the principal must be fully committed." But then why does direct revelation mechanism allow for no commitment in the later part of the paragraph? What exactly does "if this is not the case" mean here? What exactly does commitment mean here?