An HSM or Hardware Security Module is a device used to securely sign and submit data through the Bacs and Secure-IP services. The software identifies an HSM via its slot ID and not via the IP address. This is how a cluster is able to function.
When you create a cluster, it is given one slot ID, so if one HSM fails then the software is still able to authenticate through that slot ID with the remaining HSM's within the cluster.