The One inch punch is a technique from Chinese martial arts which was popularized by Jeet Kune Do founder Bruce Lee.
It is commonly believed that the One inch punch skill comes from Kung fu style of Wing Chun, however, it is present in many styles of Southern Chinese martial arts. As a general rule, Southern Chinese martial arts rely mostly on hand techniques from very close quarters (as opposed to Northern Chinese martial arts which focus more on kicking techniques from medium to long distances). Because the Southern styles martial artists were often fighting nose-to-nose with their opponents, they had to learn a way to deal out punishing blows even while almost touching their target.
The One inch punch is a skill which uses Fah jing (explosive power) to generate tremendous amounts of impact force at extremely close distances. There are several different opinions regarding where this power is generated in the body. As with most Chinese martial arts, the power for this attack comes from the Dan tian. The dan tian is an area about three inches below the navel from which all power derives. When performing this one inch punch the practitioner stands with his fist very close to the target (the distance depends on the skill of the practitioner, usually from 0-6 inches). Then in one explosive burst, the legs root, the waist turns, the ribs expand and the arm extends through the target. It is crucial that the entire body move in unison, or else the power will be limited. The target in such demonstrations vary, sometimes it is a fellow practitioner holding a phone book on the chest, sometimes wooden boards can be broken.
Source = Wikipedia
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