When a man squeezes his balls so that the scrotum bunches up slightly and takes on the appearance, shape, and texture of brains.
Carl pulled his balls out and showed everyone his rat brains.
When a dude stretches his scrotum tightly to one testicle and displays it. He could put it right next to a sleeping person's face, for instance. You could also just tightly wrap it to two testicles.
Dude, get your rat brain out of my face.
When wearing shorts you pull out one nut to show a suprised victim, thus ratbrain.
"Hey man wanna see a ratbrain?"
"Huh, a what...oh ewwww!!!"