Tettsui Hizo Uchi is a technique in Kyokushin karate. It translates to 'Hammer Fist Side Strike.' This technique involves delivering a powerful strike with the hammer fist to the opponent's side. The term 'Tettsui' refers to the hammer fist, and 'Hizo Uchi' indicates a strike to the side. It is an effective technique for targeting the ribs, kidneys, or other vulnerable areas on the opponent's torso.