At first the view uses a normal pan gesture that triggers a Core Animation when it is let go. Then, we switch to dynamics, add an attachment to a pan gesture, gravity, collision detection and make the view springy so it bounces. Now, we can grab the view, throw it against the bottom, and watch it bounce. If we want to change the parameters of the behavior, they are all represented as simple properties in a physics simulation.
See the article in Smashing Magazine for the code and explanation.