This is the case when the source and target repositories of a project specify different values for the SubDir option, as in

[project]
source = svn:test
target = darcs:test

[darcs:test]
subdir = darcs

[svn:test]
subdir = svn

When this happens, tailor uses rsync to copy the modifications done by the application of any single changeset in the SourceRepository WorkingDirectory to the TargetRepository's one.

This has some advantages:

1. It has an unconfirmed reputation of being a better method, since it seems to work even with TrickyCases
2. Some people think its a cleaner way
3. It allows migrating changesets between two different and independent repositories of the same kind
4. It's the only way to migrate from a DarcsRepository to a GitRepository (due to DarcsGit support)

On the other hand, it:

1. Doubles the disk space usage
2. Takes some more CPU and I/O slices