Adding a new rule to the codebase creates a fork in the blockchain that node operators have to accept. A hard fork signifies a radical change in which the new additions to the chain and the nodes on that chain are no longer compatible with the old chain. A hard fork differs from a soft fork in that some node operators continue to operate on the old chain, causing two blockchains to emerge, with the chased. This has happened in the past with Bitcoin and why you can still find a small number of node operators running what are now different cryptocurrencies.