Kakutō uchi (literally: “wrist joint strike”) or the bent wrist strike uses the “ball” at the end of the ulna to strike an opponent under the chin. While this is not a powerful technique, it is incredibly fast, and thus makes an ideal atemi. This motion has defensive applications as well; these will be discussed elsewhere as kakutō uke.
[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 the wrist is moving the shoulder, and not the other way around. For maximum speed, keep your hand relaxed throughout this movement. Your hand should resemble a swimming jellyfish. The ball of your wrist will strikes underneath the opponent’s chin, like an uppercut. When done 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]
When Mr. Miyagi made young Daniel-san paint his fence, the upstroke was kakutō uchi.