asr has two methods it can use to restore an image to a volume: it can either copy items in a file-by-file mode, or it can directly block-copy the image. As the latter method does not need to go through the OS filesystem, it is much faster, typically being limited only by the speed of the drives or network connections involved. But in order to use this faster method the .dmg file involved must first be checksummed, and in some cases the files re-ordered inside the image. This is also done by the same tool.
The graphical Disk Utility uses asr processes in the background to perform restores.[1]
