A way to break the chain and create a new one. I was making a joke and using a very loose definition of 'blockchain'[0], so shoot me.
0. "a data structure that carries some quantum of payload and a reference to the previous block"
I guess DAG with a merkle tree data structure would be more appropriate. Blockchain should be reserved for DAG's that use Nakamoto Consensus (or similar) to determine the longest chain.