Here the idea was that you have one large ball with a smaller ball hanging from it.  The large ball travels across the screen and the smaller ball must then inherit that motion.  In doing so, you have to take into account the ideas behind intertia.  As the large ball begins to move, the smaller one wants to stay where it is, therefore it gets dragged behind a little bit.  Once the large ball stops though, the smaller ball, now moving, wants to continue to move.  It does so, while being limited by being tied to the large ball and by gravity.  So the small ball is going to slowly come to a stop, more or less in a pendulum motion.