When someone is presenting using PowerPoint, where is your eye typically forced to go? Most likely, you will find yourself staring at the screen (no matter what is on it!). This is one of the aspects of the "tyranny of PowerPoint".
If you are presenting, however, and you want to make an important point - or add drama to your presentation - try using the "B" key.
In Slideshow mode the “B” key toggles between the presentation and a blank (black) screen (it doesn’t matter if you use upper or lower case – simply pressing the key executes the toggle).
Using the "B" key provides you with the opportunity to direct attention to yourself (and away from the PowerPoint slides). This can help you to make critical points, move to a whiteboard for an ad hoc drawing or to develop a concept, or otherwise break up the standard PowerPoint slide-after-slide-after-slide presentation!