If you animate a character, aparrently c_trace always looks at his first frame only. So if in the first frame there's a leg at a certain place, but now he's walking, and the leg's somewhere else, it would still hit the player, unless you do some updating ever frame. (am I right?... or did I misunderstand something?)
This system is a new way of tracing so that the player's hit in the right way even when he's not in his first frame, but playing an animation.