Why does --backup imply the delta-transfer algorithm, which in turn implies -u?

Question

From http://www.jveweb.net/en/archives/2011/02/using-rsync-and-cron-to-automate-incremental-backups.html#jveweb_en_027_06

Using the --backup parameter enables the delta-transfer algorithm, so the use of the -u parameter is not necessary.

1. How can we understand --backup implies the delta-transfer algorithm?

2. -u, --update skips files that are newer on the receiver. The delta-transfer algorithm is to actually transfer a file, but not to make a decision whether this file will actually be transferred or not. So how does enabling delta-transfer algorithm imply -u?

Note that I found neither of the two are mentioned in the manpage of rsync, but I might miss something.

Thanks.

Answer 1

Using the --backup parameter enables the delta-transfer algorithm, so the use of the -u parameter is not necessary.

Wrong! https://xkcd.com/386/ (someone is wrong on the Internet).

How can we understand --backup implies the delta-transfer algorithm?

It doesn't. The delta-transfer algorithm is the default.

So how does enabling delta-transfer algorithm imply -u?

It doesn't.