This refers to moving or keeping something out of the readers’ view. Think rape or child abuse as two common topics. While the story may focus on these acts and the characters may deal and react, the readers never read the action directly.
It’s a method to explore or explain a situation that is both violent and unsettling, even necessary to the story but uncomfortable for the writer. It requires the readers’ mind to fill in the blanks, something we all do and which is why we imagine the worse of the situation. It allows the writer to tell the story without tuning readers away…everyone handles a topic differently. Everyone has a different level of what’s horrific.
However, keeping any of these two examples on page is also a strong writing tool in delivering a powerful punch. A statement you (a) don’t want ignore or (b) need to write for you.
Going off page allows for something to maintain its strength and not become overused by repeating. What’s deliberately left off page has less chance of being gratuitous and brushed aside. It allows the writer to tell the larger story without focusing on the act.
Try writing something off page and then on page, which works for you…don’t fall into the trap of thinking you need to show/tell your author everything.