SCVMM takes a Hyper-V snapshot of the running virtual machine. This will create a differencing disk for each VHD connected to the VM. All disk write operations from that point forward go into the differencing disk. The original base VHD is no longer changing since it is in a read-only state.
With the base VHD in a read-only state, SCVMM starts to transfer the file from the source location to the target location using BITS. This represents the bulk of the data that needs to be transferred and the VM remains running during this transfer. In addition, QSM does not depend on storage types, and the user is free to select any storage destination that is accessible to the Hyper-V host.