In a technology I work in, Frame Relay, the calling end of the telecommunication data circuit is called the master and the called end the slave, using this same thought process. In a similar vane, for backup purposes you can have two slaves, a slave with backup and a backup slave. In fact, you can have a whole series of backup slaves. This is used say when you have one remote terminal trying to connect to two redundant data centres. You would connect to one, and if that one was down, then you would connect to the other.
So it has nothing to do with whips and chains if you hear me talking of masters and slaves. Though I would be amused if you did think of whips and chains.