This feature, combined with the graceful shutdown feature, will enable data nodes to come back up and start serving quickly. This is likely a command line option for data node, which triggers it to look for saved state information in its local storage. If the information is present and reasonably up-to-date, data node would skip some of the startup steps.
Ideally it should be able to do quick registration without requiring removal of all blocks from the date node descriptor on the name node and reconstructing it with the initial full block report. This implies that all RBW blocks are recorded during shutdown and on start-up they are not turned into RWR. Other than the quick registration, name node should treat the restart as if few heart beats were lost from the node. There should be no unexpected replica state changes.