diff --git a/doc/updating-modules.rst b/doc/updating-modules.rst
index 5482533..55a4200 100644
--- a/doc/updating-modules.rst
+++ b/doc/updating-modules.rst
@@ -8,11 +8,23 @@ To update CMake modules you need to run the ``boostrap.py`` script::
   cd cmake
   python bootstrap.py ..
 
-The CMake modules are not fetched or updated at configure time
-or build time.
+The CMake modules are not fetched or updated at configure time or build time.
+In other words, if you never re-run ``boostrap.py`` script and never modify the
+CMake module files, then the CMake modules will remain forever frozen.
 
 
 How to pin CMake modules to a certain version
 ---------------------------------------------
 
-Write me ...
+Sometimes you may want to avoid using the latest version of a CMake module and
+rather fetch an older version with the hash ``abcd123``. To achieve this, instead
+of::
+
+  [coverage]
+  source: https://github.com/scisoft/autocmake/raw/master/modules/UseCodeCoverage.cmake
+
+pin the version to ``abcd123`` (you do not need to specify the full Git hash, a unique
+beginning will do)::
+
+  [coverage]
+  source: https://github.com/scisoft/autocmake/raw/abcd123/modules/UseCodeCoverage.cmake