Breaking the diffraction barrier!

I was under the impression that the resolution of any optical microscope would be limited by diffraction; apparently, it is not so. The latest PNAS carries an article about breaking the diffraction barrier in flourescence microscopy using photoswitchable proteins. Here is the abstract. By the way, the paper is classified (also) under the key keyword nanoscopy.

I am yet to understand the concept of how they break the diffraction barrier; that might answer the question as to whether this method can be used for the study of non-biological specimens. The paper does contain micrographs of grooves on a glass specimen obtained using their microscope. However, those are obtained by filling the grooves with fluorescing proteins (by absorption). In that case, absorption of a fluorescing medium will be a necessary condition, and, I do not know if metallurgical samples, for example, absorb any such dyes. Any pointers?