Being a junior diver, I was very confused with all the talk here regarding BP&W. In my beginner class they never mentioned it, so I decided to do a little search. On this forum there isn't a good, comprehensive description of what the thing exactly is (for long time I thought it is something you just add to your current setup).
For a good andunbiased overview go to
Backplate and wing - Wikipedia, the free encyclopedia
to learn more. It certainly helped me.