You've probably used the picture in picture functionality of FaceTime at some point. As Jordan points out, it's a good example of how to implement UIFieldBehavior in an app. This is a great post to really focus on understanding how it works and how you can use UIFieldBehavior too.
Using UIFieldBehavior, we can designate areas of our view to apply certain physics effects whenever an item enters into them.
UIFieldBehavior by Jordan Morgan