Offloaded Data Transfer (ODX) lets Windows Server 2012 request that the SAN actually perform the move or copy actions directly, bypassing the host, which removes any performance hit on the Windows Server host and allows the SAN to perform the actions much faster.
Most of the major SAN vendors are working with Microsoft to support ODX in their SANs, which will allow any file move or copy operation that goes through the file service APIs such as command line tools, Windows Explorer and even Hyper-V to be actioned directly by the SAN. Some vendors that have tested and will have available ODX solutions include Dell, HP, IBM, NetApp, EMC and Fujitsu.
Some key scenarios where the speed difference would be major would be moving a large virtual machine (VM) or even creating a new VM from a template on the SAN--the process can now take seconds instead of minutes. This same technology can be used between separate SANs that have support for cross SAN ODX.
If you are using a SAN with Windows Server 2012, definitely look for ODX support by the vendor, as it will give better disk performance and save resources on the actual host.