So this is a little easter egg type feature for users of Engage: Publish.
If you want to embed YouTube videos in your Engage: Publish articles you can do so with standard HTML, or you can use this new feature added to the 5.0.2 release of Publish.
If you put [_youtube_:_######_] replacing the #### with a video ID such as pJbSDiaK8co, Publish will replace that with the proper HTML (Also remove the underscores, I had to put those in there so Publish wouldn't replace the text above)
Example
This is the first of what will eventually be a token framework in the Publish module. Through the use of Tokens you will be able to configure how a particular article/category displays on your website. Stay tuned for more insider Publish tips.
One catch with the YouTube token, Publish will currently only replace on YouTube token per article, in a future release it will work for more than one per.
There is a bug in 5.0.2 relating to this YouTube token, you must put it in lowercase for it to work.