diff --git a/Documentation/gitattributes.adoc b/Documentation/gitattributes.adoc
index bd76167a45..da773e2924 100644
--- a/Documentation/gitattributes.adoc
+++ b/Documentation/gitattributes.adoc
@@ -784,6 +784,17 @@ with the above configuration, i.e. `j-c-diff`, with 7
 parameters, just like `GIT_EXTERNAL_DIFF` program is called.
 See linkgit:git[1] for details.
 
+An external diff driver replaces the patch Git would otherwise
+produce for the path: Git runs the command and shows its output in
+place of its own.  Output features that post-process Git's diff do
+not apply to the driver's output; word diff, function context (`-W`),
+`--color-moved`, and coloring all act on Git's builtin diff, not the
+driver's output.
+The driver is consulted only when Git generates a textual patch.  The
+summary formats (`--stat`, `--numstat`, `--shortstat`, and
+`--dirstat`), `git blame`, and `git log -L` do not run it and
+continue to use Git's builtin diff.
+
 If the program is able to ignore certain changes (similar to
 `git diff --ignore-space-change`), then also set the option
 `trustExitCode` to true.  It is then expected to return exit code 1 if