NIST specified SHA-2 hash functions with truncated output. Those hashes use different initialization values than SHA-256 or SHA-512. SHA-224 is based on SHA-256. SHA-384, SHA-512/224 and SHA-512/256 are all based on SHA-512.
Although I have seen loose comments on why truncated SHA-2 functions use different initial values, I haven't seen any strong reasoning. The only thing that I can formally find is a quote from RFC 3874: "A 224-bit One-way Hash Function: SHA-224", section "1.1. Usage Considerations:
The use of a different initial value ensures that a truncated SHA-256 message digest value cannot be mistaken for a SHA-224 message digest value computed on the same data.
But that quote doesn't list any specific attacks nor is it part of a security review of SHA-2.
Can anybody explain which attacks are prevented by choosing different initial values for the SHA-2 variants?
I'm specifically looking for answers that indicate how the initial values help mitigate the attack. More importantly, I'm looking for authoritative answers, that is: answers that can point to a security evaluation or proof of SHA-2 on this subject.