You draw a great crossbow back by turning a small winch. Loading a great crossbow is a full-round action that provokes attacks of opportunity. Normally, operating a great crossbow requires two hands. However, you can shoot, but not load, a great crossbow with one hand at a -4 penalty on attack rolls. You can shoot a great crossbow with each hand, but you take a penalty on attack rolls as if attacking with two weapons. These penalties are cumulative with the penalty for firing the crossbow one-handed.