This is the part of calibration that separates the pros from the amateurs. Relying on visual brightness and contrast settings introduces too much variability into the system. For one thing, I think the Huey is assuming you'll have a really bad monitor (or really bad eyes); on the screen where it asks if I see three rings, I actually saw five, and there's no way to adjust the controls to see fewer. Setting the brightness and contrast values to 75% and 50%, respectively, is a suboptimal solution.
The directions can be a bit vague, as well. First you set contrast (the white/gray circle), then you set Brightness (the black/gray circle). But until you realize what each screen is for--they're not labeled as such--the instructions for Contrast are confusing. What does "it" refer to in the second sentence?
Once you've adjusted the brightness and contrast, you place the calibrator on the display so it can measure color and gray values. Pantone includes a cleaning kit for LCD monitors so the calibrator's little suction cups adhere better.
This is the screen the software runs through; the calibrator sits over the big cigar-shaped spot, which runs through a sequence of color and gray values. This part of the process takes a surprisingly short time--less than 5 minutes.
Huey can poll your environment for changes to lighting at regular intervals, but it's optional. If you have a window nearby (lucky you) or move lights around (as I do, to view print samples), then it comes in handy.