A hand gesture, involving the act of two people each making a fist with one hand, extending their arms, and pressing them together, knuckle to knuckle, in a gesture of fraternity and brotherhood.
Afterwards, it may or may not be "blown up" by immediately withdrawing each fist and extending the fingers fully, as if the rock has just exploded. Talented individuals may even attach sound effects to this act of blowing up.
After my cross examination, I returned to my seat and my classmate said "Give me the rock." Doing his due dilligence, he didn't forget to blow it up, and said "Way to go, Perry Mason"