I am not a breeder. However, I think for the more conscientious breeders, using lines from progenitors that tended to throw certain qualities- are what they seek in molding the next generation.
For top tier sport dogs, it is as if you cannot get away from certain names not too far back in the pedigree. Is this blind luck? For lesser known working dogs, there are also qualities that tend to surface using particular lines, reliably.
I think perhaps breeders don't speak of this in any great detail because it is the art of what they do- like a working chef giving out the intricate details of his most beloved dish would be unlikely. I also think of breeding much like the artistry and science of winemaking, and we know not all wines are to everyone's tastes.
The interesting thing for enthusiasts of the German shepherd dog is that we have a solid history of documented quantitative and qualitative information on our beloved dogs to turn to.