How to use it
Use a shape like an arrow to connect two areas of an interface such that the arrow visually "spills" from one area into the other.
When to use it
If you have two areas that have a parent-child relationship to each other - like a multi-layered navigation - you can use an arrow to visually connect them. If it's navigation, the arrow can line up with the navigation item that the user selected. This way, it's creating a direct visual relationship between the item they selected and the second navigation area that is "created" as a result.
This approach stops the divider between the two areas of navigation feeling too separate from each other.
What to watch out for
Note that the two arrows in the example below are alternatives - it's best to use one or the other, not both.
The shape doesn't have to be a triangle, but it's probably helpful if the shape suggests movement in some way.
The shape will naturally get in the way of layout because it takes up space that a straight-line divider wouldn't.