This command creates a Shutter button. A Shutter button is normally opaque, but becomes transparent when first clicked. It then becomes opaque when clicked again, etc. When a slide is reset, Shutter buttons revert to being opaque.
By chaining shutter buttons together in various ways, you can arrage it so that clicking on an area of the screen reveals just that area and hides all the rest,
or so that slicking on one item of a list reveals that item but hides all others, for example.