When using the Sharpening tool. There is a slider for masking. If your want to know what is actually happening when you move that slider, press and hold the "option/alt" button. This will display exactly what is being done.
If you use the "option/alt" button while adjusting the detail, it will look like this.
The "Amount" of sharpening will look like this.
This is an important tool to use in Lightroom 3. When making any adjustments press and hold the "option/alt" button and see if any other options exist. They make using some of these arbitrary sliders much easier and more visual.