diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -793,7 +793,13 @@ class manifest(revlog.revlog):
         return self._newmanifest(d)
 
     def readfast(self, node):
-        '''use the faster of readdelta or read'''
+        '''use the faster of readdelta or read
+
+        This will return a manifest which is either only the files
+        added/modified relative to p1, or all files in the
+        manifest. Which one is returned depends on the codepath used
+        to retrieve the data.
+        '''
         r = self.rev(node)
         deltaparent = self.deltaparent(r)
         if deltaparent != revlog.nullrev and deltaparent in self.parentrevs(r):