How Does the 51% Attack Exploit a Weakness in the PoW Consensus Mechanism?

A 51% attack occurs when a single entity or coordinated group gains control of more than half (51%) of the network's total mining hash rate. With this majority, they can effectively block new transactions, reverse their own recent transactions (double-spending), and prevent other miners from completing blocks.

The "weakness" is the economic vulnerability that the security relies on the majority of miners acting honestly.