That has to do with the Contrast Sensitivity of the Human Visual System (see followup post). But it is often misinterpreted that those medium resolution regions (say 4-6 cycles/degree), where our eyes are most sensitive to, are the only ones that need to be pushed for a better looking image. Instead, we are given an opportunity to (also) boost those other spatial frequencies that our eyes are less sensitive to, the higher spatial frequencies, the smaller details at 60 cycles/degree. Those spatial frequencies, or detail sizes, is exactly what Topaz Detail offers control over, and then some (e.g. based on color contrast, or by deconvolution sharpening).
(http://www.telescope-optics.net/images/eye_contrast.PNG)
For those not fully up to speed on this Contrast Sensitivity Function (CSF) stuff, this link (http://www.telescope-optics.net/eye_intensity_response.htm#frequency) will give some pointers with a fairly good summary of the issues for viewing, and even though it's from a Telescope/Astronomy point of view, it's equally relevant (if not more) for our regular image output situation. Output is limited in contrast, compared to our eye's and display's capabilities, so the CSF even more important when we produce printed output.
When we shoot an image, there are already things like Veiling Glare that will reduce the original scene's contrast on the sensor, and squeezing a high contrast image content into a limited range contrast output (display, or worse a print), will require proper tonemapping (e.g. a good implementation of Clarity) or else the image will look flat and lifeless.
But then we still need to manage the Human Visual System's (HVS) contrast sensitivity which drops when detail gets smaller and smaller until the contrast that our eye sees becomes so low that we cannot detect any detail. If we were to only boost the spatial frequencies (or angular detail if we also account for viewing distance) where the HVS does best, then the image would only gain a bit of 'pop', nicer contrast for those size details we can already see the best anyway.
But when we would boost the contrast of those small details where the HVS can barely see any detail anymore, around 60 cycles/degree (or an average 20/20 vision limit, or something like 290 PPI at reading distance), then the image will look sharper overall, because we can see more small detail despite our eye's loss of contrast there. Not only will we see e.g. more/smaller tree branches against the sky, but we can also start to see more surface structures like e.g. veins in leafs, despite the loss of contrast of our Visual system's limitations.
Our HVS also loses contrast sensitivity at the lower end of resolution. And because that determines the overall impression of contrast, we can manipulate that in our favor. Also remember that medium to low spatial frequency detail will become higher resolution detail when viewed from a distance, so if we boost those spatial frequencies then the output will remain looking sharp from a distance as well. The peak of our eye's contrast sensitivity also changes with age (very low a birth, increasing with age, and then falling again as we get older) and it varies with the level of illumination at which we view an image. So if we know that illumination levels will be poor, we need to compensate a bit more.
Finally, we can also reduce contrast if we want to de-emphasize certain detail sizes, e.g. because the image gets too 'busy' and might distract from the main subject / composition. Topaz Detail allows to apply a mask if we want to only apply local enhancements, or an overall opacity if we want to subtly reduce the resulting effects a bit. It also allows to "Apply" a given stage of adjustments, and then keep on building more effects on top of the earlier ones while still in the plugin. It also offers Snapshots, e.g. to compare between different approaches while in the plugin environment, or to capture different stages of progress should one want to backtrack just a few steps at once, or it allows to Undo per latest adjustment steps.
Cheers,
Bart
P.S. Jim Kasson's blog has a nice post (http://blog.kasson.com/?p=5676) on the subject as well, and a follow up post that describes the differences between Luminance and Chrominance Contrast sensitivity. The latter cannot be addressed with Topaz Detail directly, but may be dealt with by indirectly targeting local Luminance as a substitute, or by using the Tone adjustments that allow to change relative color contrast, or by boosting saturation locally. But Luminance detail is most important for Human visual resolution.