This concept of “Yao Ying Da Ying, Mo Ying Da Ying” is one of the core concepts of Wing Chun thinking. I would like to unpack it so it is fully understood. First, we must understand Wing Chun gate theory (see my article on this). Also we need to understand a little of the theory of Yum Yeung. In this context, we are differentiating the actual body mass of a structure, its trajectory or line of attack and its function as “form” (the tangible, visible, clearly expressed movement) from the perceived threat of actual movement and predicted impact power. At inception, movement and power must come from intention. Intention is therefore “shadow”. Once intention is expressed it becomes “form”. Thus the perception of impending attack which is uncertain in that it is perceived as potential rather than perceived as actuality differentiates “form” and “shadow”. This is easy to demonstrate but a little hard to express in words but I think my meaning is clear.
Yum Yeung theory describes “form” as substantially existing (Yum in this context), and intent as formless and as not existing (Yeung in this context). It thus differentiates actuality from potentiality. Form together with what gives it perceptual reality (substance) is Yum whilst intention which has not yet become form or is not yet clearly perceived is Yeung. For example, my palm, fist or foot is Yum (actually concretely existing) whilst the speed of motion and impact power they can generate is Yeung (existing only in a very ephemeral, abstract sense as I strike). Actuality and potentiality. Potential and kinetic energy. This helps us understand the distinction between “form” and “shadow”.
The saying “Yao Ying Da Ying, Mo Ying Da Ying” means if there is a structure (“form”), strike it, if there is an impression (“shadow”), then strike at that. Hence, if we are able to observe or feel a structure entering a gate we close the gate and counter. We then “strike form”. We strike actuality. If we aren’t able to observe or feel any definite structure (“no form” or “formlessness”) but sense one, or our peripheral vision senses movement, we close the gate and strike. We strike potentiality. We don’t feint in Wing Chun and we treat the feints of others as if they were attacks. We also treat the attack intention of others as the attack it actually is.
Thus, implied in this saying is the notion of pre-emptive attack once it is apparent our opponent intends to attack us. An experienced Wing Chun person will be able to pick up on subtle changes in body language, proximity, voice tone and movement which precede an attack. Of course if the intention to attack is verbally announced then it is even easier to predict the attack’s imminence. So, impending attack or an attack which is not clearly seen but picked up by our peripheral vision or experienced intuition is to be seen as “shadow” rather than “form”. Thus, it is said we ought to strike shadow.