It reads like they use a special pixel shader to color each pixel of a texture in a more of less unique color based on when the pixel (or maybe block) was scheduled to be shaded (I'm guessing here based on the atomic counter bit). This yields a texture that shows the order in which pixel shaders are run on a texture. This seems to be different for different GPUs and architectures (unsurprisingly) and also seems to give insights into how certain things are implemented.

But that's all just a bit of guessing. Someone else might be able to give an actual explanation.