The third set of Wing Chun is called "Bil Jee," which literally translates into "Thrusting Fingers." According to master Wong Kiu, the real interpretation of the name "Bil Jee" is "Pointing to the Target." The idea of the third set is that once you have lost your center, then you can regain it again with movements from the third set.
The third set logically follows the second set by introducing several new fighting concepts. It introduces Judo type of foot sweeps, defense by means of direct finger counterattacks, the concept of offbalancing or throwing techniques, the hook punch, double arm grabbing, the use of vertical elbowing techniques, the Searching Hand (Man sau), the Gan sau, and most importantly; the use of a whipping sort of power which uses violent body rotations to add additional power to all deflections and strikes.
It has been said that the center to center actions of the first and second set are derived from those of the snake. The side to center actions of the third set, in particular the elbow and Man sau movements, are derived from those of the graceful crane.
The sticking hands training cannot be complete without the techniques from the third set. This means, without the third set, you will encounter problems during the sticking hands training which do not have easy solutions. Therefore, someone who is proficient with the third set skills will have an advantage over someone who does not know this set.
The techniques of the third set are a little more dangerous when applied, and therefore requires more control. Finger sweeps, finger thrusts, elbow attacks and foot sweeps are harder to control than normal punching, palming and chopping attacks. This is perhaps the reason why the third set was rarely taught (in the interest of safety). Although the third set is a more advanced set than the second set, it does not mean that you can do away with the second set. For example, one counter to the third set vertical elbow movement is the second set horizontal elbow movement.
The movements of the third set teach you to not only have the force of the head of the snake in your hands, but also to have the force of the snakes body in your body. The double grab and turn in the third set for example, is a violent vibrational force like a dog shaking off some water or like a startled cat.
The Wing Chun Kung Fu Digital Library