The SLAVEOF command can change the replication settings of a slave on the fly. If a Redis server is already acting as slave, the command SLAVEOF NO ONE will turn off the replication, turning the Redis server into a MASTER. In the proper form SLAVEOF hostname port will make the server a slave of another server listening at the specified hostname and port.
If a server is already a slave of some master, SLAVEOF hostname port will stop the replication against the old server and start the synchronization against the new one, discarding the old dataset.
The form SLAVEOF NO ONE will stop replication, turning the server into a MASTER, but will not discard the replication. So, if the old master stops working, it is possible to turn the slave into a master and set the application to use this new master in read/write. Later when the other Redis server is fixed, it can be reconfigured to work as a slave.
A note about slavery: it's unfortunate that originally the master-slave terminology was picked for databases. When Redis was designed the existing terminology was used without much analysis of alternatives, however a SLAVEOF NO ONE command was added as a freedom message. Instead of changing the terminology, that would require breaking backward compatible in the API and INFO output, we want to use this page to remember you about slavery, a crime against humanity now but something that was perpetuated throughout the whole human history.
If slavery is not wrong, nothing is wrong. -- Abraham Lincoln