diff --git a/contrib/catapipe.py b/contrib/catapipe.py
--- a/contrib/catapipe.py
+++ b/contrib/catapipe.py
@@ -6,6 +6,14 @@
 # GNU General Public License version 2 or any later version.
 """Tool read primitive events from a pipe to produce a catapult trace.
 
+Usage:
+    Terminal 1: $ catapipe.py /tmp/mypipe /tmp/trace.json
+    Terminal 2: $ HGCATAPULTSERVERPIPE=/tmp/mypipe hg root
+    <ctrl-c catapipe.py in Terminal 1>
+    $ catapult/tracing/bin/trace2html /tmp/trace.json  # produce /tmp/trace.html
+    <open trace.html in your browser of choice; the WASD keys are very useful>
+    (catapult is located at https://github.com/catapult-project/catapult)
+
 For now the event stream supports
 
   START $SESSIONID ...
@@ -24,7 +32,7 @@
 
 Typically you'll want to place the path to the named pipe in the
 HGCATAPULTSERVERPIPE environment variable, which both run-tests and hg
-understand.
+understand. To trace *only* run-tests, use HGTESTCATAPULTSERVERPIPE instead.
 """
 from __future__ import absolute_import, print_function
 
diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1084,6 +1084,7 @@
         env["HGENCODINGMODE"] = "strict"
         env["HGHOSTNAME"] = "test-hostname"
         env['HGIPV6'] = str(int(self._useipv6))
+        # See contrib/catapipe.py for how to use this functionality.
         if 'HGTESTCATAPULTSERVERPIPE' not in env:
             # If we don't have HGTESTCATAPULTSERVERPIPE explicitly set, pull the
             # non-test one in as a default, otherwise set to devnull