The automation is done using a "heartbeat" cable that is connected to the two servers. As long as there is a "pulse" or "heartbeat" from the main server to the second server, the second server will not initiate its systems. There may also be a third "spare parts" server that has running spare components for "hot" switching to prevent down time.
The second server will immediately take over the work of the first as soon as it detects an alteration in the "heartbeat" of the first machine. Some systems have the ability to page or send a message to a pre-assigned technician or center.
In some systems failover is intentionally not entirely automatic, requiring human intervention. This is called "automated with manual approval", as the activity is automatic once approval is given.
Failback, conversely, is the process of restoring a system/component/service in a state of failover back to its original state (before failure).