Okay, consider it covered. The Opacity Mask system sort of operates like the Light and Dark contone mapping with images, except it maps transparency instead of colors. A quick example is:
1. Import a bitmap, a photo would be best.
2. Create a rectangle that covers it. Then apply a linear gradient fill to it, and on the Infobar, make the gradient repeat several times, from black to white, to black, and so on.
3. copy the rectangle to the clipboard.
4. With the image selected, choose Edit>Paste Opacity Mack>Paste in Place.
You'll see that the copy of the rectangle now maps black to transparent in the photo, while white maps opacity, and shades of black are intermediate shades of transparency. Put a shape behind the photo to really see what's been done, and use the Layers gallery to detach an opacity mask.
More comprehensive coverage to come real soon.