A high degree of difficulty manuever done in mens' gymnastics on rings. A gymnast begins from an upright position called a straight body press with his arms straight down and his body vertically aligned. In a feat of incredible strength, he opens his arms away from his sides until they are perfectly horizontal. The shape his body makes is a perfect cross, one of the most beautiful images in all of athletics.
If the gymnast has good form, the transition from straight body press to the iron cross should be smooth and controlled with absolutely no ring sway. His arms should be completely horizontal, and his face should be placid and calm, masking the painful stresses his shoulder joints feel.
A gymnast must have tremendously strong shoulder and back muscles, the deltoids and trapezius. The shoulder joints have to be intact. It is difficult if not impossible to perform the Iron Cross once the shoulder's been injured. This manuever favors young, strong gymnasts.
I am unaware of any woman gymnast who's performed this routine in judged competition yet, but this is an anomaly that will be overcome with time.
A short history of the rings event of men's gymnastics can be found at:
http://www.gymmedia.com/ghent2001/appa/rings/history_ri.htm