None. For the recursive resolver it is effectively the same as a TTL expiration. The next request initiates a recursion which adds a small number of milliseconds to the response time and is recached.

I would imagine it does some kind of atomic swap, where it's just force updating an existing cache and not actually flushing it.

The cache probably gets re-filled again pretty quickly, if you were to spam this endpoint it'd be more of a problem but I would assume/hope there's rate limiting on it.