How Does a 51% Attack Exploit the Consensus Mechanism?

A 51% attack occurs when a single entity or group gains control of more than half (51%) of the network's total mining hash rate or staking power. With this majority control, the attacker can manipulate the transaction order and prevent new transactions from gaining confirmation.

Critically, they can reverse their own recent transactions, effectively enabling a double-spend of their coins. This undermines the trust and immutability of the blockchain.