Attach the slings to an overhead, fixed bar. Stand on a step or bench so you are able to reach the level of the bar. Place the upper arms inside the slings and place the hands on the top part of the slings for support. Step off the platform so that the body is hanging in the air.
Using the lower abs, slowly roll the knees up towards the chest. While holding the knees up in the air, move them over to the right, and then all the way back over to the left. Repeat the movement, mimicking the motion of a windshield wiper.