"The requirements for the CrossFit pull-up are simple yet tough to execute. The pull-up begins from a hang at full arm and shoulder extension and ends, regardless of grip, with the chest pressed tightly to the bar.
Pulling the chest to the bar is very hard but encourages full "opening the chest" and pulling the shoulders back or "closing the back." In the early stages of developing a pull-up it is acceptable to bring the Adams's apple to the bar instaed of the chest, but leading with the shoulders forward rather than back is a fault that needs to be fixed long before you are goin to get to 30 pull-ups." -Greg Glassman, CrossFIt Journal, 2003