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 the ulna up along your centerline. While superficially, this can be used to redirect an opponent's techniques upward, it is better 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, 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. Your hand should resemble a swimming jellyfish. The ball of your wrist will contact the opponent. 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 uke.