When a man cups his thumb and forefinger around the top of his testicles (up toward the choder aka taint). He then applies a gentle amount of pressure to make the testicles bulge. When you look at it just right- the "seam" that separates his left and right testicle makes his gonads look like a palm-sized brain. Hence the term "Baby (for small) Brains."
Tom: "Gosh Bob is really drunk."
Paul: "I know, I hope he doesn't take his pants off again."
Tom: "That guy gets fruiter the more he drinks."
<enter naked Bob>
"Hah hah! Look guys... <sqeezes his nuts> BABY BRAINS!!"
Tom & Paul: "Ah, man not again. Put your pants back on homo, and grab us a beer."