Lower the rings so they are a bit lower than your waist. Grab the rings with both your arms and have the rings in front of you. Bend your back and knees.
Start the movement by lifting up one of your legs while pushing away from the ground with the other leg. Use the momentum to get your both legs up pointing to the ceiling. At that moment slow the movement down and slowly lower your legs in the opposite side.
Reverse the movement in a similar manner - push off the ground with one of your legs while lifting the other. And again - slow your movement when on top and steady get back to starting position.