Preemption is a feature that allows one node to relinquish activity to a peering node when the health values are equal. This functionality can be enabled through configuration (preferred CLI command). If one of the nodes in the redundant pair is enabled for preemption (has a preferred keyword configured), then this node, upon a bootup, takes over the activity from the existing active peering node with the same health value.
In a setup with two nodes with the same health value and no preferred command is configured on either node, the currently active node remains active and does not relinquish its activity status to a just booted up node unless the health value is changed.
If the health values are different between the peering nodes, preemption is automatically disabled, and the activity is driven solely by the health value.
A freshly booted node that is about to take over the activity from an existing node (because of preemption), waits for all the flows to be synchronized and transferred from the currently active node. Only after this transfer is complete, does switchover occur.