Kakutō-uchi (literally: “wrist joint strike”) or the bent wrist strike strikes opponent under the chin with the “ball” at the end of the ulna. While kakutō uchi is not powerful, it is incredibly fast, and thus makes an ideal atemi. The defensive applications of this motion are discussed elsewhere.
[Photo of the ball of the wrist]
Start with your hand in front of your center. Imagine a rope is tied around your wrist, and it is being pulled up. The elbow does not bend, but the shoulder rotates. Imagine that your wrist is moving your shoulder, and not the other way around. For maximum speed, keep your hand relaxed throughout this movement when done correctly, your hand should resemble a swimming jellyfish. When performed correctly, this technique will not enter the opponent’s field of view, making it a surprise.
[video of kakuto uchi from the front and side]
Kakutō-uchi is the upstroke of Mr. Miyagi's "paint the fence" technique.