Kakutō uke (literally: “wrist joint block”), the bent wrist block, is the defensive application of kakutō uchi. Kakutō uke drives the “ball” at the end of your ulna up along your centerline. While this can be used to redirect an opponent's techniques upward, it is best to use kakutō uke as a wedge technique.
[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 does not 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 resemble a swimming jellyfish. The ball of your wrist contacts the opponent. When done correctly, this technique wont enter the opponent’s field-of-view, making it a surprise.
[video of kakuto uchi from the front and side]
Mr. Miyagi made young Daniel-san paint his fence because the upstroke was kakutō uke.