Wing chun as a fighting art is based on relatively basic concepts, however people tend to over complicate it because they have not established the basic preconditions necessary to start chi sau. Essentially we want use our arms (or legs) to either deliver our body mass into our opponent, or to use those limbs to use the body mass to protect us. Chi sau is an exercise in doing this simultaneously so the opponent is always having to deal with a force pointing towards their centre. The wheels fall off for people when they are only able to utilise a small amount of their mass, which tends to lead to an exercise in aligning in straight lines behind the fist: very powerful in one direction but this requires constant adjustment which affects your balance and provides an opportunity to the opponent. My believe is that this is a very common method, hence the prevalence on youtube of videos showing people chasing hands. If your focus is on hands, you have missed the point of the exercise as you risk the opponent adjusting their body mass to develop overwhelming power.
Once a student can relax their joints sufficiently, it becomes relatively easy to release a tan sau and even roll into bong sau. In a sense I think that people accept that this easier to do than fook sau because they can see something happening, they can feel it and therefore are satisfied that their actions are responsible for the effect on the partner. With a fook sau, my experience is people get lost in the desire for movement/reaction and forget what the training objective is. If two inexperienced students are put together it will quickly develop into one person either holding their fook sau down to restrain the partner or alternatively their wrist being dragged across their body as they seek to 'stick' to the opponent. The answer for this is always return to the training objectives. First pay attention to your relaxed structure (let go), second point your mass at the whole opponent. We need to remember that the goal is more than restraining their forearm, if you set yourself up properly first you not only restrict their ability to produce power, you allow yourself to move your own joints with less effort and deliver your mass.
It all sounds simple but it is not easy. I have heard it told back to me many times how difficult it is to concentrate all of the time, essentially having to be in the moment throughout a movement is hard (at first at least). You can cheat, you can throw out punches, lean, push and pull, but that is all risky guess work which is only likely to work on a compliant or smaller partner trained in the same drills as yourself. The satisfaction comes in being mindful through the movements, allowing it to happen based on the basic instructions you give your body, without the micro management of constant forearm alignment.
It is impossible to teach what a fook sau is in writing, practical experience is the best teacher. A tip I can give is do not get hung up on the names of techniques, bong, tan, fook, jut etc, basically your hand is either on the inside or outside and that in itself will determine what response your body will decide is appropriate.
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