So LFJ, can you elaborate about that "abstract idea"?
Otherwise, regarding bong sau, in our lineage we generally use it in response to opposing force. Contact with heavy force bends our arm into bong, as Yak described, much like a flexible piece of rattan or bamboo.
This is an example of what my old sifu called "objective" versus "subjective" technique. "subjective" technique requires no contact with the opponent's limbs to initiate. you perceive an opening or a threat and you move of your own accord to respond. Since the response is initiated by you, "the subject" it's "subjective".
An "objective" reponse is one that occurs in response to an outside or objective force. In our WC we seek to be as "objective" as possible so that ideally our opponent's energies directly initiate our responses. His punch literally creates our tan or bong by bending and pushing our arms into that position. Such an objective response is desirable for several reasons:
First, it uses the opponent's energy and doesn't crash force. Borrowing the force this way conserves energy and makes it possible to effectively handle powerful opponents.
Second, as the opponent initiates our response by physically pushing or bending our arm into position, the response is very fast. Since our response occurs as a result of a direct mechanical linkage, there is no "reaction time" lag while your nervous system processes stimuli received.
Finally, if you are responding "objectively" to the energy that you receive rather than to visual cues, you are not succeptible to visual fakes.
Now all this may sound a bit abstract, but it is just our understanding of: 'Loi lau hoi sung, lat sau jik chung."