Fight situations are unpredictable. You cannot predict what or how things will occur in the future, all you can do is be prepared to handle to what is thrown at you. Predefined, choreographed moves are unusable since there are so many variables involved e.g. enviroment, number of people and thier ability/experience, shape, size, strength, any weapons, and particularly the unpredictable human actions. The only way to handle such unpredictability is to react according to the moment without consciously thinking what to do; focusing on your opponent and being in control with via the conditioned, disciplined instinctive reactions you can develop. Wing Chun uses Chi Sau to develop these aspects.
Chi Sau is unique to Wing Chun. It is not a method of fighting but a method of training. Its ultimate aim is to train a practitioner to react instantaneously in a fight situation. The intention of Chi Sau is to eventually help the practitioner get the job done faster with minimum effort and maximum effect. A Wing Chun practioner uses Chi Sau as the basis to launch Fan Sau (returning hand) overwhelming an attacker with barrages of simultanious controll and attack, which is continued until the opponent is totally neutralized.
To sum up, the main purpose of Chi Sau is to instill an ability to go into autopilot, which is achieved through constantly practicing techniques in different ways, against different opponents and with varying non-premeditated situations and responses. Chi Sau develops the ability to breakdown both an opponents’ attack and defensive actions.