The default overwrite policy compares the modification date and size of a file to avoid comparing the actual file contents, which depending on the file size could take a long time.

A checksum (or hash) is a short character sequence computed from a given input (e.g. a file). A checksum is, with high probability, unique for different inputs, and can hence be used to compare the inputs.

Checksums would be convenient to compare files if they would readily be available. In order to compute the checksum of a file, the whole file contents need to be read first. So computing checksums of the files to compare is usually at least as expensive as comparing the file contents directly, or even just copying the source to the destination.

On the other hand, checksums can be explicitly saved for files copied to the destination with the backup strategy Incremental, link unchanged files to verify the backup later.