This was a change implemented in Hard Fork 26.
Blocks on Hive become irreversible as soon as a super majority of the currently scheduled block producers vote on the block. This happens within a few hundred milliseconds, even before the second block is produced.
Hive uses Delegated Proof of Stake (DPoS) as its consensus mechanism. One Block Irreversibility is the same as a fully confirmed block on a Proof of Work (PoW) blockchain such as Bitcoin.
Fast Settlement Times
This is an important concept for merchants. When accepting payment using either of the base layer Hive coins, either HBD or $HIVE, we can see how the transaction is irreversible within a few seconds. If we compare that to Bitcoin, the top PoW chain, we see how this can take more than 10 minutes under the best case scenario.
If we contrast this with the existing banking system, we see how it is no contest. Within certain countries, it can take a couple days before a transaction is fully settled at both the account and bank layer. This gets worse when we add international transactions. That can often take a week or more before settlement occurs.
With Hive, this could be done in a 3 second plus a few hundred milliseconds. The transactions are placed into blocks that cannot be changed.