If you don't get the feature from the video...essentially you can create a "content block" as they call it. That content block embeds another document within iA Writer.
For example, you have a master document that has a bunch of content blocks that are kind of like links to the other mini documents. If you update one of those mini documents and then preview the master document...all the changes made in any of the mini documents are immediately reflected in the master.
It is a simpler way (in my opinion) to manage long documents that have multiple sections.
Ulysses does something like this where you can "glue" sheets together...but it's not as intuitive or user friendly as what iA is doing now. I still love both apps and did most of my masters degree writing with Ulysses. It's awesome to see competition in this app arena and to have the folks with iA Labs try to push the world of markdown forward.