Set-up: Grasp two gymnastic rings with an underhand grip. Make sure the rings are set high enough that your feet will not touch the ground at full hang.
Lift: Begin by hanging from the rings and bracing through your core. Now raise your legs straight out in front of you so they are parallel to the floor, contracting your abdominals and surrounding muscles while minimizing flexion in your lumbar spine. Hold your legs in this position for the duration of the exercise. Now lift yourself up until your chin is over the bottom of the rings, trying to get as high as possible. Cue yourself to pull your chest to the rings to ensure full height. Descend all the way to full extension as this will engage more of the shoulder stabilizers and muscle mass of the back - this will help in maintaining health in the shoulder girdle.
Emphasis: Do not swing your body up to the rings. It will change the movement pattern and should functionally be considered a different exercise. While useful in other contexts - swing and momentum are inappropriate here.
Optional weighting: A weight belt or weight vest will allow for external loading to make the exercise more difficult.