In some embodiments the patent details an application switch icon on the phone call interface which would return the user to the original application; however, Patently Apple notes that this interface has likely been replaced with the new multitasking bar.
Another embodiment detailed is the status bar which which appears at the top of the screen when on a call and in an application other than the phone app.
The switch application icon is a visual cue for the user. When a user activates this switch application icon by touching it with their finger, the iPhone replaces the menu of application icons with the controlling iPhone GUI.
The patent is titled "Portable electronic device with graphical user interface supporting application switching" and Apple credits Senior VP of iPhone Software Scott Forstall and team members Greg Christie, Stephen Lemay, Marcel Van Os and Imran Chaudhri as the inventors of this patent.