Hyper-V R2 added a new capability called Cluster Shared Volumes (CSV). This feature provides the much-desired ability to handle individual Virtual Hard Disk (VHD) files as individual items for failover. Prior to CSV, as you probably know, you had to fail over an entire disk, rather than individual VHD files within it.
Fast-forward to today. The technologies that let CSV-enabled volumes operate still require one cluster node that's responsible for the coordination of file access. This cluster node is called the coordinator node, with each individual LUN having its own coordinator node.
That node can be any of your cluster hosts, with each host having an equal chance of being given the job. While this responsibility doesn't come into play often—typically, Hyper-V interacts with its disk files directly, not necessarily through a coordinator node—it's important for certain types of actions. One of those actions is copying VHD files to a LUN. Hyper-V transparently redirects the file copy through the coordinator node.
This redirection obviously means that VHD file copies can take longer if you initiate them from servers other than the coordinator node. So always do heavy VHD file work so from the coordinator node to save yourself time.