Guess I'm behind the curve because this is the first time I've heard of this nonsense, but if you also have not previously heard about the challah segulah here goes:
If you are baking challah between after Sh'kiya on Wednesday and before Shavuos, and are making enough to make the bracha "hafrashas challah", please email me so that you can be part of the 40 women segulah.No other explanation was given in the email, but a quick web search turned up an essay by Chabad-nik Rav Yitchak Ginsburgh, gematria extraordinaire. Apparently, the mitzvah of challah is the primary rectification for the nation of Israel abandoning the Land of Israel as a result of the sin of the spies. Since they wandered for 40 years in the desert, I guess having 40 women baking somehow helps to correct that sin. Plus, the gematria of Rachel = 40 when you use the ordinal count of each letter (there is almost always a way to get what you want with gematriot if you are creative enough!), and the mitzvah of challah relates specifically to Rachel.
The amount of superstition that has permeated mainstream RW Orthodoxy is absolutely astounding. But occasionally this obsession with segulot gives birth to a gem of an idea: Rav Ginsburgh suggests that they open up a bakery inside Rachel's tomb!