The longest valid chain wins. People can't add invalid blocks to the chain and have them be accepted by the rest of the community that is not in on it. The pow

The longest valid chain wins. People can't add invalid blocks to the chain and have them be accepted by the rest of the community that is not in on it. The power of >50% is that you can keep some (valid) blocks on the chain, like some payments to a service provider, for as long as you want and have them accepted by the community because that's the longest chain that everyone's seen, and collect the service for that payment. Then whenever you want release another chain, that's longer and doesn't include the payment blocks, invalidating the payment. The rest of the 49% of the users can still agree that someone has 5m, nobody's gonna take that away from them. But they're not going to be able to agree on any payments on the chain, because the 51% could invalidate them at any point.