diff --git a/doc/en/usage.txt b/doc/en/usage.txt
index de3a501f4..f068db403 100644
--- a/doc/en/usage.txt
+++ b/doc/en/usage.txt
@@ -44,8 +44,11 @@ Specifying tests / selecting tests
 Several test run options::
 
     py.test test_mod.py   # run tests in module
-    py.test somepath      # run all tests below path
-    py.test -k string     # only run tests whose names contain a string
+    py.test somepath      # run all tests below somepath
+    py.test -k stringexpr # only run tests with names that match the
+                          # the "string expression", e.g. "MyClass and not method" 
+                          # will select TestMyClass.test_something
+                          # but not TestMyClass.test_method_simple
 
 Import 'pkg' and use its filesystem location to find and run tests::