diff --git a/doc/preferred_content.mdwn b/doc/preferred_content.mdwn
index b81c45fbf7..f6071ea353 100644
--- a/doc/preferred_content.mdwn
+++ b/doc/preferred_content.mdwn
@@ -169,6 +169,8 @@ So when is `unused` useful in a preferred content expression?
 
 Using `git annex sync --content --all` will ensure that all keys, including
 unused ones, are examined and the preferred content expressions followed.
+Similarly, `git annex sync --content --unused` will only look at the unused
+keys.
 
 The git-annex assistant periodically scans for unused files, and
 moves them to some repository whose preferred content expression