-is not used and the whole file is sent as-is instead. This may be
-useful when using rsync with a local machine.
+is not used and the whole file is sent as-is instead. The transfer may be
+faster if this option is used when the bandwidth between the source and
+target machines is higher than the bandwidth to disk (especially when the
+"disk" is actually a networked file system). This is the default when both
+the source and target are on the local machine.