You would take turns striking at one another; the hitter can hit as hard as they like, and the receiver cannot move away to the dodge the blow. Generally played by hitting the arm, but anywhere is acceptable. Although the game is viewed as INCREDIBLY stupid, the winner is seen as "badass" due to the violent nature of the game.
Guy2: Shit, I can't even move from last time...
Marissa: Shot of vodka?
Alex: No, how about Fireball.
Marissa: That'll do.
Alex: Damn peer pressure.