Visor

Visor completely covers the slide area with a gray color rectangular shield when you are in PowerPoint slide show mode. The gray cover can be dragged around to show the covered area and change the opacity level.

In your PowerPoint presentation, select the slide you want to add visor.
Click . The visor mark will appear at the lower left corner of the slide.
Switch to slide show mode when you are done setting all the slides with visor.
Click and drag the gray cover to the location on where you want position it.
At the upper right corner of the visor, click to change the opacity level of the visor, to close the visor, and to save the visor position and opacity level.