The Actions that will be available in Extender 2013 can be called using several different events on a Window, Form, or the Shared Actions, including the following:
·         Open Form
·         Close Form
·         Open Window
·         Close Window
·         Enter Field
·         Change Field
·         Exit Field
·         Save Table
·         Delete Table
·         Shared Actions
Depending on where the Action is being setup and if Extender Standard or Enterprise is being used, the Event Types available may differ.
What these trigger points will allow you to do is run an action when the trigger point is met.  So for example, you could have an Extender Window open as soon as a specific GP window is opened.