More information is necessary here, there are a number of different ways to configure redundancy but I will assume you are referencing a single vNIC with a group of VLANs that is set to Failover Enabled.
This would mean that at the UCS level when we detect a link down for any reason, the vNIC will lose the edge port on the switch that it is pinned to. It will commence the process of re-pinning, it will analyze available uplinks to locate an uplink that satisfies ALL of the configured VLANs on the vNIC. At which point it will pin to said uplink.
That is the basic process, please flesh out your question if that did not cover what you are looking for.