Hi Milfoil,
I'm not sure how they work really but Wikipedia is a good read
http://en.wikipedia.org/wiki/Biorhythm
The cycles have different durations:- Physical is 23 days long, Emotional:- 28 and Intellectual:- 33. When you use your birthdate and how many days you have lived, you find out where in the cycles you are. Yesterday I was 18 days into the Physical, so almost at the end. The cycles run for the duration, then they start again. They are supposed to be in tune with your body. One site I went to mentioned chakras.
The site I linked doesn't finish off the calculations to make the chart - it's a little bit more involved but then you can start plotting the chart out to see whether the flow line is going up or down. I have a book coming from Amazon that should show how to finish up with the chart.