diff --git a/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/consumer/RemoteInputChannel.java b/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/consumer/RemoteInputChannel.java
index c22ba964ae19a7..7d6c98d73e9f26 100644
--- a/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/consumer/RemoteInputChannel.java
+++ b/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/partition/consumer/RemoteInputChannel.java
@@ -784,7 +784,9 @@ public void onBuffer(Buffer buffer, int sequenceNumber, int backlog, int subpart
         boolean recycleBuffer = true;
 
         // The first buffer from the producer proves the upstream reader is registered and the
-        // connection is live; release any recovery-side awaiter. On later buffers this is a cheap
+        // connection is live; release any recovery-side awaiter (see requestRecoveryBufferBlocking).
+        // During recovery this first arrival is typically the upstream's EndOfOutputChannelStateEvent,
+        // which is guaranteed and passes the zero-credit gate. On later buffers this is a cheap
         // idempotent no-op (the latch count is already zero).
         upstreamReady.countDown();