Full Idea
B-properties supervene on A-properties if no two possible situations are identical with respect to their A-properties while differing in their B-properties.
Gist of Idea
Properties supervene if you can't have one without the other
Source
David J.Chalmers (The Conscious Mind [1996], 1.2.1)
Book Reference
Chalmers,David J.: 'The Conscious Mind' [OUP 1997], p.33
A Reaction
Personally I would have thought that if this condition is achieved, then we could go on to say B-properties supervene on A because A is causing them. We shouldn't be shy about this. Personally I think the Bs are necessary.