Two clusters agree that one cluster will borrow resources from the other, taking control of the resources. Both clusters must change their configuration to make this possible, and the arrangement, called a “lease”, does not expire, although it might change due to changes in the cluster configuration.
With this model, scheduling of jobs is always done by a single cluster. When a queue is configured to run jobs on borrowed hosts, LSF schedules jobs as if the borrowed hosts actually belonged to the cluster.