The grip you have when you grab some guy by the testicles, hence having a "handful of nuts". Agonising for men, it is a source of endless amusement for onlookers and passers-by. Another form of this term is "going the squirrel", which means that someone is attempting to grab someone else in a squirrel grip.
Person 1: This guy picked a fight with me the other day, but I got him in the "squirrel grip" and he sure changed his tune quickly.
Person A: This guy attacked Sophie in a dark alley, so she "went the squirrel" on him and escaped. Poor bastard'll be singing high-C from now on.