The unnaturally pronounced oscillation of the ass during motion such as walking/running. Can be seen in models strutting down the runway, toned high level athletes, etc. Similar to the way a cheetah's shoulder blades oscillate when stalking.
Damn, that girl's got some assillation going on. Look at that booty go!
Those speedskating suits don't leave much to the imagination! You can even see the assillation!