Front punch begins chambered high and tight on the ribs, palm up. Fist leaves the side, arm slides along the ribs palm up, until the elbow leaves the side. Opposite arm pulls to chamber. Try to keep shoulders square to target at impact.
As it leaves the side, the fist rotates toward thumb to face floor, completing its rotation as punch reaches full extension.
Point of contact with target is with the first two knuckles.
Note that at point of contact there should be 2-4 inches of extension left in the arm, so that punch penetrates the target.
Practice this penetration on pad or makiwara, since dojo training with partner is usually pulled, resulting in poor combat habits. Keep the shoulders down and relaxed.
Power is generated through relaxation and explosive movement, along with forward motion of body as stance moves (lunge punch) or through rotation of hips (reverse puch).
Some instructors teach to tense arm on impact, though such tension can restrict chi/qi/ki flow.