The On Hide trigger allows you to trigger an action whenever an object is no longer visible, whether it is by a specific Hide action or that the page on which it appears is no longer on screen.
Note:
This latter case is not terribly useful for visual events (it will happen out of view) but can be useful for saving information to a variable or disk file and thereby saves information collected on the page without any further intervention from the user.
Add the On Hide trigger to your Actions Organiser, as described in Creating and Editing Actions.
Click on the Actions tab in the Actions dialog to select an action or actions to add to the trigger.
Go to Choosing an Action to list all of the actions you can add to your trigger.