reverse punchline

When a comedian tells a joke or a story, they say the actual funny part at the end and the comedians voice trails off and acts like it’s nothing making it even funnier due to a complete dependence on the audience paying attention, usually ended with a dead stare.

Reverse punchline
I had a nightmare

It was a nightmare about standing in front of a group of people
I said I had nightmares

And they said, nightmares, what are those?