diff --git a/repo2docker/app.py b/repo2docker/app.py
index 1fbc0dc9..c09ee9f4 100644
--- a/repo2docker/app.py
+++ b/repo2docker/app.py
@@ -354,7 +354,8 @@ class Repo2Docker(Application):
     target_repo_dir = Unicode(
         "",
         help="""
-        Path inside the image where contents of the repositories are copied to.
+        Path inside the image where contents of the repositories are copied to,
+        and where all the build operations (such as postBuild) happen.
 
         Defaults to ${HOME} if not set
         """,