it was a link right at the start of all the forum posts, there was a discusion about it. If you google biorythms and gender selection or somethng like that you should get lots of info, there is even a site that bases its whole gender selection package on this theory, you send them money and they send you a chart based on your own biorythm calender telling you when to bd for your desired gender.
i checked out the theory with a bit of my own research and basically if i understand it right if your emotional and physical line is in the neg its good for a girl and in the pos is best for boy. It seems to be linked with acid/alkaline balance.
You can find websites that give you a biorythm chart free on line, and then check dates yourself. my fav one i found is the link below but thee are lots. If both you and dh are in the correct position its even beter for the sway.
Going to spend some more time researching but its an interesting one.
http://www.cycletourist.com/biochart/biodoc.php#
Hope this helps, let me know what you think.
cheers