First there is the scale of abstract versus concrete. Second there is the concept of overview versus details.
If one looks at how a design progresses over time, regardless if one follows a waterfall or agile process, one moves both from abstract to concrete and from overview to details, but they are not the same thing. And I think that agile and waterfall puts different emphasis on on to make these transisions, and where to start and end (topic for another blog post!)
Going from overview to details is simple, the more you know about the problem and the solution the more information is added. think of going from a sketch where you only elaborated on the key classes to understand the system to a class diagam with all classes that are implemented. Using a map as analogy; you increase the scale to see more details, the coastline gets more details, or you see all roads and not only the big ones.
Going from abstract to concrete is also about adding details, and I think that is where the confusion begins. But the details that are added are not of the same type as in the more abstract representation. Here it could be adding all attributes of classes or ports to components. You don't get this information by adding more classes or structure components. In the map analogy it would mean adding indvidual houses on a map which previously only had roads, lakes and forests. It is a new type of information which you cannot deduct (transform) from the more abstract representation.
