- Pressing "B" during a slideshow turns the screen black
- Pressing "W" makes it white
With one button, you bring back all the attention of the audience to you, and you can deliver an important message without slide support.
UPDATE: Bert Decker pointed me to one of his blog posts in which he suggests deliberartely injecting black (or white maybe) slides into a presentation. After you press "B" to black out a slide in a live presentation, you have to make it appear again to continue with your story. Inserting black slides in the presentation does not require to bring the audience back to a point you already made.