Good Ambiguity: An Example

One of the authors whose story did not make the final round of judging gave me permission to quote a line from it that I greatly admired. 

His story opened with a family barbeque. The main character is an adult woman probably in her late 20's or early 30's. The opening paragraph ends like so:

While the [family members ate and drank], I tried not to think about sleeping with my sister's husband.

I liked that opening a lot, and I liked it even more an instant later when I recognized I wasn't sure if the narrator had already slept with her sister's husband, or was just imagining doing it. 

Now in my opinion, one of the problems with this story was how quickly that question got answered. The next sentence settled the matter, and I'd have preferred to see the tension stretched out a bit longer. The point is, though, that here we have an author who is being ambiguous in a way that helps a story, instead of hurts it. Usually it's a serious problem when we read a sentence and we're not clear on what it means. Writing is all about communication, right? 

Yes, and in communication, what is not said can be even more powerful than what is said. The above line says quite a bit, and teases us even more with what it holds back. That's compelling writing. If the rest of the story had impressed us as much as that line did, it would have made the final round of judging.