Tate tsuki, the vertical punch, is a specialized move used to strike the torso from close ranges. Some karate styles, such as Isshin-ryū, use this punch exclusively. When your opponent is closer than arm’s reach -- close enough to grab you -- turning your punches over will cause you to raise your elbows, resulting in weak punches which use only the shoulder, and not the whole body. Also, this exposes your ribs to attack.
Unlike a normal punch, tate tsuki does not turn over, and it does not fully extend. A tate tsuki remains at your side, and ends when the elbow is above -- and not in front of -- the hip. The resulting technique is reminiscent of the Rock’em Sock’em Robots (which like Goshin-Jutsu, also originated from Erie).
[Video of tate tsuki from the front and side.]
Because tate tsuki does not extend or turn over, it will be weaker than a regular punch. Recall that power does not reside in the arms -- it lies in the hips, legs, and core; the product of efficient motion with synchronized breathing. Tate tsuki can only work with proper biomechanics, so learning to throw a powerful tate tsuki is a lesson that carries over to all your other techniques. This is why tate tsuki needs to be a regular part of your heavy bag routine.