diff --git a/synapse/app/synchrotron.py b/synapse/app/synchrotron.py
index 07d3d047c6..dbaa48035d 100644
--- a/synapse/app/synchrotron.py
+++ b/synapse/app/synchrotron.py
@@ -242,6 +242,9 @@ class SynchrotronTyping(object):
         self._room_typing = {}
 
     def stream_positions(self):
+        # We must update this typing token from the response of the previous
+        # sync. In particular, the stream id may "reset" back to zero/a low
+        # value which we *must* use for the next replication request.
         return {"typing": self._latest_room_serial}
 
     def process_replication(self, result):