diff --git a/src/calibre/gui2/bars.py b/src/calibre/gui2/bars.py
index 8bae9d65d1..d586bfe1c3 100644
--- a/src/calibre/gui2/bars.py
+++ b/src/calibre/gui2/bars.py
@@ -277,6 +277,13 @@ class BarsManager(QObject):
             bar.init_bar(actions)
 
     def update_bars(self):
+        '''
+        This shows the correct main toolbar and rebuilds the menubar based on
+        whether a device is connected or not. Note that the toolbars are
+        explicitly not rebuilt, this is to workaround a Qt limitation iwth
+        QToolButton's popup menus and modal dialogs. If you want the toolbars
+        rebuilt, call init_bars().
+        '''
         showing_device = self.location_manager.has_device
         main_bar = self.main_bars[1 if showing_device else 0]
         child_bar = self.child_bars[0]