It's based on "patches" (which was what the original comment was referring to).

A Darcs patch is somewhat like a diff, in that most patches will describe a sequence of text to be removed and added from specific points in each file.

But: Darcs patches add:

- The ability to work out precisely how to apply the same patch in a different context in a reproducible way (or to refuse to do so if it can't do it safely) - More "semantic" kinds of change such as replacing all occurrences of a token in a file, or renaming a file. In both cases this will merge cleanly and reproducibly with other changes to the same file.