A student asked this question last week in class. It's a very good question.
It is an easy defense to just step back when somebody is attacking but at some stage, you're either going to: - run out of space - get hit; - have to defend and counterattack.
So that being the case, you may as well learn how to defend and counterattack properly, so you only step back when you have to. The problem with stepping back all the time, is that you actually don't learn much at all!
Chi Sao is not a static exercise, so you are expected to move around and move your opponent around, to try to create openings, to destabilize your opponent, to corrupt their centreline. Chi Sao is also an opportunity for experimentation - learning how to absorb energy and return it to your opponent - you can't do that if you always move out of exchange range.
Under pressure, does your structure crumble and the only way to maintain it is to retreat?
If that's the case you may have a problem with sensing pressure and responding to that pressure appropriately. When an opponent presses forward, your hands/arms feel that pressure or energy and within milliseconds, you have to make a judgement call on how you're going to respond. Just backing away does nothing for developing your 'response to pressure' reflex.
Sometimes we step back (exchange step) to change feet or change sides to attack on a different angle. If that's when you step back, that's great - but stepping back just to keep yourself out of reach of your opponent is not a long term successful strategy. Sooner or later you'll get caught and get hammered. Far better to learn how to deflect and counterattack than just retreat.
If your structure does crumble under pressure, it is possible that your core (your dantien area) is not being engaged. The muscles, tendons and fascia in this region that connect with and control your legs and upper body, the psoas, serratus, obliques and spinal erectors need to be engaged to maintain your structure. The dantien region connects everything together. Your hands and feet connect via the dantien - it's like Grand Central for your muscle and tendon connections.
Practise your forms slowly like a Qi Gong exercise and use your mind to focus on and build power and connectivity in your dantien engine room. This will help your Chi Sao enormously.
(Zhan Zhuang, or 'standing post' can also be used as a good training aid to help you focus on building your internal connections and structure)
The point of Chi Sao is to 'stick' and control your opponents centre and balance, so you can strike when the opening presents itself - or you create the opening yourself - then exploit it. Feel the pressure from your opponent and in accord with Wing Chun principles of absorb, deflect, protect your centre, corrupt your opponents centre and respond. Frequent practice will help you develop this skill.
So step back ONLY as a means to counterattack or attack on a different angle.
Chi Sao is a great exercise and a fun way of learning important fighting skills, So enjoy, manage your structure, fine-tune your 'response to pressure' reflex, keep forward pressure on and have fun!
Sifu Paul McCarthy