This task sequence variable can be used to configure a post action for a task sequence. To configure this, follow the next steps:

  • Open a task sequence in the Task Sequence Editor.
  • Add a Set Task Sequence Variable –step to the task sequence.
  • Fill in as Task Sequence Variable SMSTSPostAction and, in this example, as Valuecmd /c shutdown /s /t 0 /f.
  • Close the Task Sequence Editor.

http://www.petervanderwoude.nl/post/how-to-perform-an-action-directly-after-the-task-sequence-is-finished-with-configmgr-2012/