From what I understand, the object that enters the body (such as a fence post or a knife) serves as a stopper for any major bleeding. When the object is removed, it's like removing the stopper and thus the danger of a lot of blood loss in a short time. That's why you'll see people brought into emergency rooms with knives still stuck in them, or in this case with fence posts still stuck in them. At least the offending object can be removed in a setting more suitable for handling a bleeding emergency.
I would presume that after being impaled and before that photo was taken he would try to wiggle a little, attempting to unimpale himself - and that would provide the movement around the wound.