It can be installed from pypi, or its source code can be checked out from here, or you can download a tarball from here.

Here is the project's documentation

Very brief comparison with rsync -- see also this comparison.

• rsync can be used to copy a filesystem from one host to another, or for backups using rsync's --link-dest option plus a small wrapper script
• Backshift is only for backups, and works analogously to rsync --link-dest, but it deduplicates variable-lengthed blocks of the files both intra-host and inter-host, and compresses those blocks using xz.

Backshift is not just another tar frontend. Saved data is stored in an open but created-for-the-purpose, highly compressed format that can be assembled into a tar archive to stdout on request.