Ambush

1.  The Bong Sau in this context of the form is NOT  a chosen response against a punching attack because given a choice, our hands would not be down and we can use a much more direct response against a punching attack such as a Pac Sau , Wu Sau , Fook Sau. Bong Sau is by its nature a slower weaker "block" than the other hands of Wing Chun so why would we ever choose to use it as a first response? The reason is that the context is NOT a choice, it is in the context of a surprise, an ambush, so the Bong Sau is to capitalize on the natural flinch response!
2. When we are not ready, in the context of an ambush,  the hands are down, our elbow is A LOT closer to the high line than our hands. Hence, raising the Bong Sau is A lot faster than trying to use our hands to block when our hands are low. 
3. Because it is low to high quick flinching, surprise recovery movement, the type of energy trained is very very important. Too hard and the oncoming punch can easily overpower our structure and off-balance us; too soft and he can crash right through our arms. The correct reliable force is to use a throwing type of force, NOT a pushing or hard "blocking" type of force.
4. The counterattack from Bong Sau should never be drilled as a fixed pattern. Rather, it is adaptive, the counter use completely depends  on the distance to ensure the principle of maximum directness. If he's far, we would use the Bong Sau roll overextended hand, if he is closer, we can use our Wu Sau rear hand ( shorter reach) if he is really far we can naturally use our kicks.