Alex Knight has posted one animation easing tutorial in Blend in which he also shows how the "bounce" effect works.
Animation easing is a tool built into Expression Blend to help give you a more "fluid" feel to your animation, as you can see in the above animation, the bottom cog's animation looks a lot more realistic than the simple point to point animation of the top cog.
The bounce effect is great to give you object just a little more realism, by giving the effect it's not able to just stop instantly but "bounces" around a little.