How Does a Double-Spend Attack Work in Practice after a 51% Control Is Achieved?

The attacker first initiates a transaction, often sending coins to an exchange to trade for another asset. Simultaneously, they begin mining a secret, alternative version of the blockchain where the initial transaction is reversed or never occurred.

Once the exchange confirms the initial transaction and releases the traded asset, the attacker releases their longer, secret chain. The network accepts the longer chain as the valid one, effectively erasing the initial transaction and allowing the attacker to retain their original coins while possessing the newly traded asset.