How Does the Concept of “Orphan Blocks” Relate to the Longest Chain Rule?

An orphan block is a valid block that is not included in the main, longest chain. This typically happens when two miners find a block nearly simultaneously, leading to a temporary fork.

The network eventually resolves this conflict by following the longest chain rule, and the block that is not extended by subsequent miners becomes the orphan. In a 51 percent attack, the attacker's secretly mined blocks are effectively a purposeful, private chain of orphan blocks that are then released to become the new longest chain, intentionally orphaning the legitimate chain.