I’ve had a couple pigs, and whenever I scratch them just under their chin they go all still and quiet… and bend their heads back further and further… and get this glassy stare. It’s like I’m short-circuiting their brain. How come?
They’re powerless before your noodly goodness. I can’t wait 'til they start doing your bidding. 
Cecil once wrote a bit about hypnotizing chickens which you might find applicable. I’m not sure if it’s the same thing but guinea pigs don’t seem way up there on the list of sophisticated animals that you couldn’t give them some sort of sensory overload.
Interesting.
I once had a guinea pig who distinctly did not like being scratched/chucked under the chin. On a couple of occasions when I did that he lunged at and bit me.
He was a somewhat unusual rodent, though. Fathered numerous progeny, succeeding generations of which are, to this day, probably spreading the “do-not-touch-me-under-the-chin” gene.
In my experience,
My wife behaves the same way if her back is attacked by recently sharpened fingernails. Start scratching, and she rolls the arms forward, drops the head, and begins to moan and drool. 
Some things… just… feel… good…
So what does the little cavy do when you scratch it on its back or belly?
Could this be an any way related to how a dog gets the urge to scratch when you rub or press firmly on either side of it’s breastbone?
Hypno-Toad: He purrs when I scratch his rump, and he doesn’t like me touching his belly. What I’m trying to figure out is if he actually likes having his chin scratched.
alimarx: Or why one leg twiches when you give them a belly rub?
Do humans/other great apes have anything like this? Especially ones that involve relatively major effects (things like Babinski don’t count)?
My female friends refer to the spot around the back of my head and neck as my “Off Switch.” No matter what I’m talking about, they can get me to shut up and put my head on the table of they scratch me there. 