It is called many things, but people are common with Kyusho Atemi Waza or Dim Mak. Being able to strike vital points is hard to achieve. In practice, finding the points and practicing hitting them is easy, but against a person who is hitting back, it is hard, so lots of controlled sparring is necessary.
A good way of being able to hit these points is by holding your opponent securely whilst striking. By holding, it is easier to strike and easier to locate the points. If you don’t hold, you don’t control Vitalflow and it is much harder. This is one of the reasons why there is a lot of hikite with most strikes in katas. The hikite represents holding the opponent, whilst delivering the strike to a vital point.
Vital Points include obvious ones such as eyes, groin, joints, but then there are not so obvious ones like the bottom of the shoulder where it meets the bicep or under the armpit. These are more less known points and if you would like to know more of them, I strongly recommend you get a copy of the Bubishi. It shows all the points on the human body.
To practice vital point striking, one of the best ways is to make small dots with a pen on a heavy bag, and concentrate on trying to strike these points. The beauty of vital point striking is that the strikes do not have to be very hard to hurt or do damage so don’t hit with full force on the bag. Instead, concentrate on accuracy and on using more striking surfaces rather than the fist, foot and shin, including, ridge hand, knife hand, one knuckle strikes, elbows, knees etc.
Striking vital points can be devastating, so care must be taken when practicing, and if you find yourself unfortunate to have to defend yourself on the street, ask yourself if the situation is dangerous enough for you to have to resort to vital point striking. Most of the time, simply walking away is enough to defend yourself.