I'm going to add to the mystique. I'm throwing into the mix the idea of a 2 pass BBS. Specifically when spoken of in conjunction with a type of razor or brand of blade. I personally believe that is one is truly getting a BBS in two passes (which is hard to believe) or even a DFS then it is more a result of their beard type than it is the tools.
For example, it is impossible for me to get even a good shave in two passes. To even get close I would have to shave north to south and then south to north. But! On my cheek hair grows west to east. On my neck it is angled in some areas and again west to east in others, and the opposite on the other side. So I require 3 passes minimum and this is even with my R41. And I'm sure I'm not alone in this as far as hair growth. So I can't see how a shave can be very close in two passes unless the hair is only growing in two directions. Unless when people check their shave they only feel in one direction. If I shaved in two directions I'd still have an unshaven third direction that would be rough. Not even a DFS. I never understand the 2 pass BBS.
For me the second pass is all about know the hair growth direction and not completely sticking to a true west to east pass throughout, especially in areas that will get closer east to west or even south to north. I think it's about adjusting during the second pass accordingly, to some extent.