Rack Focus :

In Rack Focus, the shot has two objects. One object is in the background and one is in the foreground. The object in the background is in focus while the object in the foreground is out of focus.

To Rack Focus, move the focus ring so that the object in the background goes out of focus while the object in the foreground comes into focus.

Or just the opposite, the background is out of focus while the object in the foreground is in focus. Move the focus ring so the object in the background comes into focus while the object in the foreground goes out of focus.

The example above is of a Rack Focus from foreground to background. The example below has a zoom in while rack focusing on the background subject.