This command creates a Mask button. A Mask button is opaque. When clicked the button itself is hidden, thus revealing whatever may be beneath the Mask. The button remains hidden until the slide is reset.
Mask buttons are useful in some situations where you want to reveal something and then be able to click buttons (e.g. Hiliter buttons) there that had previously been covered (and thus inactive). Where Shutter buttons reveal material beneath them but still remain in place, mask buttons reveal and get out of the way.
