We've just bought a Wii Fit and I'm very impressed. I've been wondering how the Balance Board would help and now I think I know how.
When you're setting up the Wii Fit it records your height (you need to input that) and weight (it measures that).
If you're doing something like a lunge you're supposed to put a certain percent (I'm guessing 50%) of your weight on your front leg. Because this leg is on the Balance Board the Wii Fit can measure the percent of your weight on that leg and provides a marker on the screen showing you if you need to put more or less weight there.
I've now used it twice in two days. I'm sure I'm going to post more about this soon...