Placing the following line inside a Drupal story node's body (I guess it also works for other content types) adds a manual teaser break:
This works fine when the story uses the "Full HTML" text format. It stops working, however, when the story's text format is switched to "Filtered HTML" - in that case Drupal simply ignores the manual teaser break and auto-breaks wherever it sees fit.
For Drupal 7, a bug report was submitted 8 long years ago, and the issue is still open today. I can only imagine that they didn't fix it because a workaround exists: A site admin can add
<!--> to the "Filtered HTML" text format's set of allowed HTML tags.
In Drupal 8 this workaround is no longer available because in Drupal 8 the "Filtered HTML" text format unconditionally filters out all HTML comments. A few days ago I submitted another bug report on drupal.org for Drupal 8, and today I decided that I'm really not in the mood for going through all my old stories and switch those with manual teaser breaks to "Full HTML". Instead I wrote a patch that fixes the issue and that is now live on this site. If you're interested you can find the patch in the bug report referenced above.