Different branches of the Wing Chun family vary widely as to how flexible and reactive their movements are. I've found that the way your branch approaches making bong-sau to be a good indicator of this. At one extreme, bong-sau is launched fiercely, almost like a block/strike, knocking aside an incoming strike. Some would call this a "hard" bong-sau, although few Wing Chun stylists would embrace this term. At the other end of the spectrum, the bong sau is created by the opponent's strike coming accross your bridge, bending your arm into bong-sau as though it were a springy piece of bamboo. I've heard this referred to as "soft" Wing Chun, but that is misleading too, since the "spring" may be very strong. In neither case is the bong-sau rock-hard and rigid nor limp and weak. The difference is more one of "subjective", ie "do you initiate the move?" versus "objective" ie "does your opponent's force initiate the move by flexing and rolling your arm?" I've found that how a Wing Chun branch conceives of bong-sau equally affects how it approaches all of it's techniques from tan-sau and fook-sau right down to stance and footwork. Over the years, I've worked with both approaches, and I've met excellent fighters from each perspective. But it's not a trivial distinction! I know that the Leung Ting Wing Tsun branch is a major proponent of the "flexible" or "let your opponent's force initiate your movement" school of thought. What kind of bong-sau do you have?