diff --git a/synapse/app/federation_sender.py b/synapse/app/federation_sender.py
index fa7b19e53a..cbddc80ca9 100644
--- a/synapse/app/federation_sender.py
+++ b/synapse/app/federation_sender.py
@@ -62,6 +62,11 @@ class FederationSenderSlaveStore(
 ):
     def __init__(self, db_conn, hs):
         super(FederationSenderSlaveStore, self).__init__(db_conn, hs)
+
+        # We pull out the current federation stream position now so that we
+        # always have a known value for the federation position in memory so
+        # that we don't have to bounce via a deferred once when we start the
+        # replication streams.
         self.federation_out_pos_startup = self._get_federation_out_pos(db_conn)
 
     def _get_federation_out_pos(self, db_conn):