Choku-tsuki (literally: “direct punch”) or front-foot punch is the karate analogue of a boxer’s jab, used as a simple and blindingly fast atemi.
To begin, enter a fighting stance. As one can infer from the name, front-foot punches are thrown with the lead hand. Push your lead elbow up and out to extend the arm, and exhale. When the fist is within 6” (15 cm) of the target, rotate your forearm from a palm-up to a palm-down position. At the moment of impact, every muscle in the arm simultaneously tenses, and then immediately releases (like a sneeze).
For optimized speed and impact, pull the elbow back to your chest twice as fast as it went out, turning the hand back into a palm-up position.
Throughout this entire process, the hand follows a corkscrew (helical) path.
[video of front-foot punch from the front and from the side.]