There is nothing else known about Master Wuxi other than his/her fame for developing the Wuxi Finger Hold sometime in the third dynasty, as Po noted in the first film.
The use of the Wuxi Finger Hold involves pinching an opponent's index finger and then flexing your pinky. Once the pinky is flexed, an explosive golden shock wave occurs. It is unknown what happens to the opponent who has the move performed on them, though the hold has built a reputation for itself as being highly advanced and dangerous.
In an interview with "Po", Scholastic News Online asked about the hardest move for him to learn. Po confirms with the Wuxi Finger Hold and states that Shifu told him (after he defeated Tai Lung with the hold) that there's really no such move; it was just a story that kung fu masters made up to scare their students into submission. In addition, Po couldn't really recall how he did it.
According to some traditional Chinese sources, the "third dynasty" that Master Wuxi had invented the hold may have been the Zhōu Dynasty.