I think the technical term is the "ionic equilibrium" technique, but the idea behind the technique is the basis of many of the diet methods. Basically, your intake of calcium/magnesium and potassium/sodium can have a molecular effect on the receptors on the outer coating of your egg (the pellucid membrane) so that they are more or less attractive to either male or female sperm.
Here is the link to a recent article that talk about it a little and shows that it is quite successful in rats. Although I find this article written a bit strange (I think it was translated from another language), it basically found that rats on a calcium/magnesium diet had approximately 80% female offspring and 20% male offspring. So they were 4 times more likely to have a girl.
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