How to use it
Add a shaped section to one of the corners of an element/area, and use it to show some other element.
When to use it
If you want to call something out to a user, you can add an area to one of the corners of an element or area of the page. I see this most often in the top right corner. It might remind people of the area on physical packaging where a guarantee or "As seen on TV" message is shown.
What to watch out for
The area does not have to be triangular like the one in the example below. It could be square, for example.