Tettsui Uchi, also known as the Hammer Fist Strike, is a technique in Shotokan Karate where the fist is clenched, and the strike is delivered using the bottom part of the fist, akin to a hammer's movement. The motion is typically executed in a downward or sideways arc, aiming to target an opponent's vulnerable areas such as the temple, collarbone, or ribs. The technique emphasizes a snapping motion, generated from the rotation of the hips and shoulders, to maximize impact and effectiveness. Tettsui Uchi is commonly used in close-range combat and can be employed to break an opponent's guard or disrupt their balance. This strike is integral to many kata and self-defense scenarios, highlighting the versatility and power of traditional karate techniques.