diff --git a/scrapy/core/downloader/manager.py b/scrapy/core/downloader/manager.py
index 697542722..59490b338 100644
--- a/scrapy/core/downloader/manager.py
+++ b/scrapy/core/downloader/manager.py
@@ -42,6 +42,7 @@ class SiteInfo(object):
         return self.max_concurrent_requests - len(self.transferring)
 
     def needs_backout(self):
+        # use self.active to include requests in the downloader middleware
         return len(self.active) > 2 * self.max_concurrent_requests