An open loop is a rhetorical device

Rhetorical device

In rhetoric, a rhetorical device or resource of language is a technique that an author or speaker uses to convey to the listener or reader a meaning with the goal of persuading him or her towards considering a topic from a different perspective. While rhetorical devices may be used to evoke an...

to instill curiosity by creating anticipation for what will come next. The device is sometimes also called a tension loop for the tension and anticipation it creates.

Short Example

"I just saw the most amazing thing!" Since the actor doesn't say what the amazing thing is, the observer is left to wonder what it is, thus creating anticipation for what the actor will say next.