Byakugan is the blood inheritance limit of the Hyuuga Clan. The Hyuuga clan was one of the founding noble families of Leaf village. The clan itself is said to be the ancestors of the Uchiha clan who possess the bloodline of the Sharingan eye. When the "Byakugan" is activated, the clan member gains an extra-sensory perception, which gives them penetrating sight and telescopic vision. This allows them to see the tenketsu; the internal chakra coils system 361 pressure points. This vision also grants the member a near 360° view of their surroundings, the higher the skill of the clan member, the further the distance from which they can sense incoming attacks. Unfortunately the vision does have a weakness, a small blind spot that extends outwards from the vertebra of the upper back.
The clan uses this ability in conjunction with their own chakra creating the Juuken (gentle fist) fighting style. The clan member can use their fingers or palms to emit chakra to flow into their opponents body's at the tenketsu points. When the hand draws close enough to strike, the clan member can stop or increase their opponent’s chakra flow. The internal chakra coils which spread throughout the body also wrap around the internal chakra producing organs. So if the internal coils are attacked, it also damages these internal organs. Because a ninja can not train their internal body, such damage can be a critical hit for even a highly skilled ninja. The tenketsu serve as the connectors and exit points for the chakra pathways. So when they are stopped by the Hyuuga clan member, the ninja greatly loses his or her ability to utilize their chakra for jutsu.