Remember the uproar when Binance CEO Changpeng Zhao insinuated that he was considering reorganising the Bitcoin blockchain? The scheme had been proposed by Bitcoin Core contributor Jeremey Rubin to to rewrite the Binance hack. Ultimately, he backpedaled from this idea, but not until the Bitcoin community expressed in no uncertain words what a bad idea this was.
Controversially, Bitcoin Cash (BCH) has done just that, and rolled back to an earlier version of the BCH blockchain, by means of a 51% attack.
A 51% attack can be executed when a group of miners control over half of the hash rate. In this position, they can do things they wouldn’t normally be able to do, like reverse the blocks on the blockchain.
The two mining pools, BTC.com and BTC.top argued that they were doing something good for the BCH community, as a hitherto unknown bug in the blockchain was allowing bad actors to mine empty blocks.
BTC.com and BTC.top were not being rewarded by taking funds themselves.