The basic concepts of a bong sao as taught to me are the following:
1- Relax shoulder (i see so many pics of people with their shoulders up)
2- Elbow leads the movement. If I were to exaggerate it my wrist and hand would twist up and down after the elbow moves. I exaggerate the movement to show people how it suppose to be, and then tell them to refine it into all one motion.
3- Your wrist should be on center line
4- Always have a wu sao (a guarding hand) up behind your bong. Also the bong sao should be soft and your wu sao should be firm (complying to the whole yin yang theory)
5- wrist is relaxed and your foreamr is slightly turned out for optimal skeletal structure.
6- There is a springy energy from the arm in the bong sao motion, it gives in a bit but always springs right back up which is focused from the elbows.
Now to break it down even further and reduce it to a really basic level we teach beginers that there are three things to consider when being attacked (three basic things that is). There is the quick jab, the medium over hand punch, and the big bruiser punch. On the very basic level of fighting your typical answers for these types of attacks are:
Pak sao for the quick jab, because you are not going to bong sao someone who is really quick and tossing jabs at you.
Tan sao for those medium paced punches (like a reverse punch).
Bong sao for those really heavy punches that come in at you with some comittment.
Each technique is structurally advantages for each attack. We teach this at a basic level to all beginers so they can get an idea of what the techs are for in combat. Of course this is just an example given and by no means the limitation of the technique. Sometimes if you break it down to the most basic levels you see it in a different light. Its kind of like the wisdom of a small child. Sometimes a child can say something thats so simple and you could not come up with that answer because you over complicated it.
The bong sao is pretty unique but not 100% exclusive to wing chun. There are a few other esoteric systems that use a bong sao like motion.