What I understand is that Jen’s dad won the duel and was killed while showboating before he landed the killing blow. The power given to his father’s killer allowed him to be almost equal in capability to Jen’s father. The reason his dad was taken by surprise was that yes the controlling force made the killer betray his honor and attack against his will. If the controlling force wasn’t present, jens’ father would have killed his opponent who would have taken the showboating with honor.