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"...used in anger" means used for the purpose it was intended for, not just in testing or rehearsal.

The analogy is to a spear or a gun; when used in anger, it is used to hurt somebody.

The new root zone key signing key will be generated at the next key ceremony. At the ceremony after that, the "same" thing won't happen, rather, the key generated will be replicated to the second site. Operational changes using the key won't happen until the key is confidently stored at both locations.

My reading of this is that it is a colloquialism indicating that that the initial actions are preparatory and in the background, with the third action being the actual cause of visible changes downstream.