Wing chun emphasizes the concept of sticking, not only sitcking in terms of maintaining contact with the limbs but also sticking close to the opponent. During the process of either attacking or defending there are moments where the limbs of combatants make contact. Wing chun generally refers to this as a "bridge" in english but this term refers to a bridge between the two bodies. This bridge can be limb to limb or limb to body/head and happens with either party striking or with the opponent attempting to grab or "hook" onto us. Actively trying to trap an opponent is a great way to get hit and should never be the goal. We chase the body, not the hands.
Chum Kiu most often translated as "building the bridge" contains striking, blocking, linear footwork, and some other movements and concepts. Nowhere in the form is there anything which would teach the student to seek limb to limb contact. Bridging limbs is a byproduct of actively attacking and a defensive focus on simultaneous defensive and offensive maneuvers for counter-striking. We chase the body, not the hands.
So limb to limb contact is incidental and trapping isn't the goal; hitting the opponent is. Whether we contact the opponent or the opponent contacts us the goal is to maneuver into a position where we can hit the opponent and the opponent is unable to retaliate during the strike. Because we are training against an actively resisting opponent any initial trap and strike will be brief as the opponent attempts to regain better position either through counter trapping, simple repositioning, or an attempt at withdrawal. Because of this reality the wing chun practitioner must be able to take the techniques they have learned and begin to apply and transition between them by feeling changes in the opponent's positioning of body and limbs through the point(s) of contact. This shares similarities with the training of many grappling arts and many of the characteristics developed are similar. Sensitivity, refelxive responses, posture, and technical focus are a few. Chi sao gives a platform to isolate the contact portion of fighting, explore techniques, and practice implementation of concepts in a resistive environment.
What's Wrong With Chi Sao
While chi sao can be a great training platform it does have some drawbacks which can be rather serious if not attended to. These are not faults inherent to chi sao but rather misapplication by practitioners and while this is not a complete list of all the errors people pake in chi sao hopefulyl it can help fix a few very common training mistakes.
Let's get one thing out of the way to start: Chi sao is not sparring, and it is not a substitute for sparring. Chi sao is a training drill which accounts for the contact portion of fighting. There are other drills and training which need equal attention. All the chi sao in the world won't save you if you can't manage space and make contact properly because you don't practice entry drills and actually spar.
Another common fault in chi sao training is the misapplication of competitive urges. While chi sao is a live and resistant training platform it is not a competitive platform. People still have the urge to win and to dominate yet while this is good for sparring it detracts from chi sao training. People try to weigh on their partner's arms to try and reduce their reaction time through fatigue. People look to make contact at the expense of body position, structure, and without any regard for their ability to power or follow up that strike. Treating chi sao as a competition only helps you waste your own time.
The last issue I would hope to see rectified in many people's chi sao is prolonged contact. You don't have time to feel out your partner from contact when they're trying to decapitate you with hooks and haymakers. You can't push and pull them around for five minutes while you learn to predict their reactions and set them up for a perfect technique. Chi sao is about learning to apply from contact and that contact is incredibly brief most often. Roll once or twice and try to execute a technique.
Chi sao can be a great tool in training but it is up to every student and Sifu to make sure that it is being used as such. Preventing bad habits from entering training is important as is remembering chi sao's place in that training.