The unsuspecting victim is standing so that their back is facing you. Approach the person, call out "knee-butt!" and knee them right in the ass bone. Hurts like hell. It is customary to call out the knee-butt warning before kneeing. When knee-butting friends it is recommended to knee with care, due to the potential anal crushing power of the move. Grabbing onto the persons shoulders can add a more visually dramatic element to the move, as well as increasing the power behind it.
Steve: "Dustin just friggin gave me the worst knee-butt of my life!"