A trigger allows you to engage with your contacts based on how they interacted with your story.
You can watch this quick demo to see how you can create a campaign with triggers.
Few things to keep in mind when you create a trigger:
Triggers can only add contacts to an existing active campaign (referred to as 'assigned campaign'), so make sure you create it before creating the main campaign
The story assignment logic for the videos in the trigger would be based on the assigned campaign.
You can have a different storyteller do the triggers. If you want a particular person only to do the trigger stories make sure you don't add any other storyteller to the assigned campaign.