Tate-tsuki, the vertical punch, is a specialized move used to punch 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 -- then you cannot turn your punches over without raising your elbows, which will result in weak punches which are only powered by the shoulder, and not the whole body. Also, raising your elbows will expose your ribs to attack.
Unlike normal punches, 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 your hip -- and not in front of your hip. The resulting technique is reminiscent of the classic 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 always be weaker than a regular punch. This is why it is imperative to drive the punch with your hips, legs, and core; power is 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 which carries over to all your other techniques. As such, it is important to make tate-tsuki a regular part of your heavy bag routine.