Once techniques have been committed to muscle memory, coordination then kicks in. Coordination allows you to carry out synchronous and simultaneous movements smoothly, with good timing and ultimately without thinking. Coordination provides the building blocks for putting techniques together in a more autonomous fashion. As coordination improves so does timing and awareness in space, and therefore it will perfect your techniques. The better coordination you develop the easier to becomes to learn new applications.
During Chi Sau, practice a high degree of coordination is developed. The awkwardness of the combined rolling seems a strange and somewhat of benign activity at first, but its significance must not be underestimated, as it provides the foundation on which to improve. With Chi Sau, coordination is developed not only using individual moves but also double and triple combinations carried out at the same time. Wing Chun can require coordination of both arm and legs, so you could be using three limbs in concurrent motion.
It is this concurrent coordination that new students find most difficult to get their head round in the beginning. Some may become despondent, but through perseverance and practice, as the body commits many an action to muscle memory, it becomes accustomed to moves a high degree of coordination. To the point that it is second nature.
Through Chi Sau training Fan Sau is cultivated. This is where coordination is taken out of the realm of repetitive sequences, to a level where synchronous actions are independent of choreographed moves, and instead rely on pure coordinated reaction at a specific moment in time. Where you handle what is present using your autopilot.
Coordination is not only about being synchronous, it is also about awareness. The more you practice a technique the more familiar you become with it. You have confidence in its versatility. As you become more accustomed to a technique, that sets the wheel in motion improvement in reflex speed and accuracy overall.