Kakutō uchi (literally: “wrist joint strike”) or the bent wrist strike strikes opponents under the chin with the “ball” at the end of your ulna. While kakutō uchi is not powerful, it is incredible speed makes it 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, which pulls it up. Your elbow doesn't bend, but your shoulder rotates. Imagine that your wrist is moving your shoulder, and not the other way around. For maximum speed, keep your hand relaxed, so that it resembles a swimming jellyfish. When performed correctly, this technique wont 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.