I found this website and several others with very similar text which describes the hypothesis.
It talks about male and female sperm carrying opposite electrical charges, and the female ovum cycling between positive and negative charges thus determining which sperm can fertilize it. Apparently this cycle is synchronized for all women everywhere and thus can be conveniently charted.
The page also talks about some research that confirmed this hypothesis, including one that said that sperm were either attracted to cathodes or anodes depending on whether they carried an X or Y chromosome. I found this bit on sperm sorting which describes labeling the sperm with a dye that attaches to DNA (and X chromosome has more DNA so will fluoresce more brightly), and then attaching a charge to each one using a single droplet of charged fluid, before sorting them. I suspect this method was confused as the sperm already carrying a charge.
The only thing that differs between male and female sperm is the DNA they are carrying. They do not only carry an X or Y chromosome, they carry fully half of the entire human genome, including one of the sex hormones. If the X and Y chromosomes have opposite charges, what charge do the rest of the chromosomes have? What happens when an X and Y chromosome are combined in a cell nucleus--do they tear through the rest of the genetic soup to snap together like fridge magnets? DNA is all made of the same stuff and the charge carried by each chromosome is the same (that is, none), so this theory can't possibly be true.
The real odds of this chart are 50%, the same as all the other outside-the-genetic-laboratory sex selection techniques.