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 with the legs extended. Bend the knees and raise them towards chest level while contracting the abdominals.
Lower knees back down and repeat.
