When the switch is powered on, the two Supervisor Engines determine which serves as the primary or active and which is the standby. Usually, the Supervisor Engine that boots first, either in slot 1 or 2, becomes the active Supervisor Engine.
But, when Hot Standby Router Protocol (HSRP) is configured, the standby preempt command enables the HSRP router with the highest priority to immediately become the active router.
If the active router fails or is removed from the network, the standby router automatically becomes the new active router. If the former active router comes back online, you can control whether it takes over as the active router with the use of the standby preempt command.