Same idea as rubber-duck debugging or just explaining things to someone else. The work of translating the idea forces your mind to marshal and walk the structures from a fresh angle and you can gain insights that were lacking.

Getting more eyeballs on that idea also helps. Both in the different knowledge and expectations / assumptions they have and in proofing how clearly the idea's communicated. Really helps reveal areas where there's ambiguity you hadn't even realized because it's not even a confusion spot you'd consider with your knowledge.