Start by squeezing your butt and then your abs to form a solid core. During the entire movement there should be no sagging in the middle. Rotate your hands, pull your elbows in tight and squeeze your shoulder blades together as you pull your armpits to the rings.
After pulling deep into the armpits, keep the tension in your body and return to the lowered position. The shoulders should not relax nor should the elbows flare out at any time.