Punches may originate from a variety of positions, depending on the circumstances the officer is in.
Rather than teach a specific starting position (like a boxer's stance or the "chamber" for martial artists), we only require the punch to pass through the area defined by the top of the shoulders to the lower chest, or the "power zone," because it maximizes the force of the punch.
As your fist passes through the power zone, it moves straight to the target, at center mass. As your arm moves forward, your fist is rotated (pronated) so that the palm faces downward at the point of impact with your target.
Proper extension requires the punch to stop before you lock your elbow joint, without overcommitting your hips and shoulders. Once extended, your fist should snap back through the power zone and to a neutral position in preparation for additional force options such as throwing additional punches.
Looking at a straight punch from start to finish, you can see that all three joints (shoulder, elbow, and wrist) come into play. Your shoulder drives your fist forward; your elbow guides the fist toward the target; and your wrist remains straight and stable to ensure proper transfer of energy and stability on impact with your target.