The second set of Wing Chun is called "Chum Kiu" or "Searching For the Bridge." In Chinese, "Bridge" refers to the arm. Searching for the Bridge refers to the art of trying to make contact with the opponent so that the sticking hand techniques can be applied.
Wing Chun divides the art of fighting into two parts. One part is the techniques which apply while in contact with the opponent. The other part is the art of trying to make contact with the opponent. Part one is much more scientific than part two. The art of trying to make contact with the opponent relies not only on speed and proper timing, but also on psychology.
The second set contains footwork and hand techniques required to make contact with an opponent. The second set teaches shifting (turning), and stepping (charging). This footwork complements the hand techniques taught in the first set.
One of the first things to notice about the second set is that it teaches one how to shift the feet. You shift on the heels of the feet while keeping the pelvis tucked in. In a shifted stance, the weight is off the front foot so that you can easily kick, deflect a kick with your foot, step or avoid foot sweeps.
Shifting (turning\rotating) serves many purposes:
The second part of the second set teaches one how to advance the stance. It teaches stepping and charging. The Wing Chun step is performed in several ways depending on the use. In the set you step with the heel of the front foot while dragging the rear foot. This kind of step keeps you solid on the ground in case the opponent attacks.
The second method of stepping is to propel yourself off the back foot using the toes. This is like a track runner starting a race. You shoot like an arrow into the opponent. The Wing Chun lunging action is similar to the lunge in Western fencing. When you are at a critical distance from the opponent, you rush in before the opponent has a chance to lift a foot to kick.
In the stepping, the feet are either on the same straight line or optimally, the heel of the rear foot is line up with the toe of the front foot. This stance is better for actual combat because it is quicker and the lead foot does not get tangled with the opponent's lead foot.