One Armed Judo

There was a 10 year old boy, who's left arm was damaged in an accident and subsequently amputated. He decided to learn judo. His Sensei (teacher) was an old Chinese judo expert. The boy learned quickly. After three months, he had learned only one move. He asked his teacher to teach him more moves. The Sensei told him that this was all he would need.Soon after, the boy entered a tournament, where he quickly advanced to the finals, where his opponent was bigger and more experienced. The boy seemed very out matched. After a long match, the opponent seemed to loose concentration. Quickly the boy took advantage and pinned what seemed to be his superior opponent. On the ride home, the boy asked his Sensei."How could I win with only one move?"The Sensei replied, "You have nearly mastered one of the most difficult moves in all of judo. And, the only defence against that move, is for your opponent to grab your left arm."