I requested additional information about Topaz Detail back when I posted my review.  I never received it, but Topaz Labs did post some info on their blog.  Eric Yang described the process as follows:

1. First, you have your original image. You now access Topaz Detail and appreciate the fine aesthetics of the pre-processing screen.

2. Pre-processing starts. Topaz Detail separates your image into one based on chrominance (color information) and one based on luminance (grayscale information). These will undergo different processing methods and will be affected by different sliders.

3. The luminance information is further internally broken down into three detail layers based on size, and a base layer. Manipulating these four layers make up the bulk of the detail enhancement functionality of the software. After this step, pre-processing ends and the Topaz Detail user interface pops up. (This is also the portion of pre-processing that runs the slowest… probably because it has the most advanced breakthrough technological algorithms.)

4. The user adjusts the large, medium, and small detail sizes, and each individual detail layer is affected appropriately. If you like, you can see what each of the four individual layers looks like. To see the base layer by itself, turn the large, medium, and small sliders to 0. To see each individual detail size layer, drag that particular one all the way to the right and set the contrast to 0 (which will switch the base layer to neutral gray).

5. The luminance layer is re-combined with the adjusted small, medium, and large detail layers as well as the base layer.

6. Highlight and shadow protection, as set by the user, is applied to the luma layer.

7. The internal luma and chroma layers are re-combined to form your startlingly good final processed image.

There have been similar manual techniques involving Photoshop’s High Pass Filter.  Topaz Detail simplifies the process, provides fine controls and saves many steps and time.  Overall, I think Topaz Detail is a great tool to add to your Photoshop toolkit.