The Cochran-Harpending theory is the cover story in the new New York magazine: "Are Jews smarter? Why the controversial new study of Jewish intelligence has everybody plotzing," by Jennifer Senior. Cochran wants to know what "plotzing" means. The magazine is supposed to be on newsstands on Monday, October 17. I don't know if the article will be online or not. (I don't know what "plotzing" means either, but I assume that's Larry David on the cover.)
For non-Yiddish speakers, to plotz means to explode, crack, or burst. Apparently metaphorical. It might be related to explode in English.
"Plotz" means something like "freak, freak out"---it means something totally takes you by surprise, you don't know how to respond, your mouth's agape, you kinda panic a little.