diff --git a/docs/source/technical.rst b/docs/source/technical.rst
index 0de1bd471..8acfadad3 100644
--- a/docs/source/technical.rst
+++ b/docs/source/technical.rst
@@ -63,6 +63,10 @@ regular subroutines
 - subroutine parameters are just variables scoped to the subroutine.
 - the arguments passed in a call are evaluated (using the eval-stack if needed) and then
   copied into those variables.
+  This sometimes can seem inefficient but it's required to allow subroutines to work locally
+  with their parameters and allow them to modify them as required, without changing the
+  variables used in the call's arguments.  If you want to get rid of this overhead you'll
+  have to make an ``asmsub`` routine in assembly instead.
 - the return value is passed back to the caller via cpu register(s):
   Byte values will be put in ``A`` .
   Word values will be put in ``A`` + ``Y`` register pair.