What the B button does is turn your screen black.
Then with no slides to distract them, the audience will focus their attention solely on you.
This is especially useful when you want to deliver an important idea, something that you want to make sure the audience remembers and understands.
It’s also useful when you move to a section like Q&A that is not covered by your slides, or if you get into a discussion about a topic that is raised during the presentation. Just blank the screen, work through the discussion, then bring back the slides and continue the presentation.
To bring your slides back simply press the B button again to bring back up your slides.
For those of you using a remote control, most remote controls have a button that does the same as the B button, so no need to rush back to the computer for a B moment.