diff --git a/tsshbatch.rst b/tsshbatch.rst
index 6979545..3e6334b 100644
--- a/tsshbatch.rst
+++ b/tsshbatch.rst
@@ -813,6 +813,38 @@
 Execution Variables
 ===================
 
+Execution Variables are actually a special case of User Defined
+Variables.  That is, they are evaluated at the same time and in the
+same manner as any other User Defined Variable.  The difference is
+that a User Defined Variable describes a *literal string replacement*.
+But an Execution Variable *runs a command, program, or script and
+assigns the results to the variable*.
+
+For example, suppose you want create a file on many machines, and you
+want that file to be named based on who ran the ``tsshbatch`` job.
+You might do this in a ``cmdfile``::
+
+  .define __WHOAMI__ = ! whoami
+  touch __WHOAMI__-Put_This_Here.txt
+
+So, if ID ``luser`` is running ``tsshbatch``, a file called
+``luser-Put_This_Here.txt`` will be created (or have its timestamp
+updated) on every machine in the ``hostlistfile`` or named with
+``-H``.
+
+Notice it is the ``!`` character is that distinguishes an Execution
+Variable from a User Defined Variable.  It is this character that
+tells ``tsshbatch``, "Go run the command to the right of me and return
+the results."  The trailing space is optional and the definition could
+be written as::
+
+  .define __WHOAMI__ = !whoami
+  
+.. NOTE:: It's important to remember that the program you are invoking
+          *runs on the same machine as* ``tsshbatch`` *itself*, NOT each
+          host you are sending commands to.  In other words, just
+          like Builtin Variables, Execution Variables are *locally
+          defined*.
 
 
 Builtin Variables
@@ -1015,7 +1047,7 @@
 
 ::
 
-  $Id: tsshbatch.rst,v 1.146 2014/12/04 22:03:42 tundra Exp $
+  $Id: tsshbatch.rst,v 1.147 2014/12/04 22:26:37 tundra Exp $
 
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