A typical use-case for replication is a combination of primary and standby server, with the standby serving as a backup which can easily be activated in case of a problem with the primary. Such an unplanned failover would normally be handled by promoting the standby, after which an appropriate action must be taken to restore the old primary.
In some cases however it's desirable to promote the standby in a planned way, e.g. so maintenance can be performed on the primary; this kind of switchover is supported by the repmgr standby switchover command.
repmgr standby switchover differs from other repmgr actions in that it also performs actions on another server (the demotion candidate), which means passwordless SSH access is required to that server from the one where repmgr standby switchover is executed.
Note: repmgr standby switchover performs a relatively complex series of operations on two servers, and should therefore be performed after careful preparation and with adequate attention. In particular you should be confident that your network environment is stable and reliable.
Additionally you should be sure that the current primary can be shut down quickly and cleanly. In particular, access from applications should be minimalized or preferably blocked completely. Also be aware that if there is a backlog of files waiting to be archived, PostgreSQL will not shut down until archiving completes.
We recommend running repmgr standby switchover at the most verbose logging level (--log-level=DEBUG --verbose) and capturing all output to assist troubleshooting any problems.