Describes the protective "cage" you form with your hand around your balls when somebody (possible a sibling or friend) wants to throw, shoot, or hit you with something. In your mind you have concluded that this projectile will cause minimal damage if it hits most places on your body; however you have also concluded if it happens to hit your sack, it could cause major damage, projectile vomiting, and possibly public shame. So with fingers spread wide and slightly curled in, and every muscle in your hand flexing to absorb the possible blow, you stand there like an idiot, and let something hit you. But it's OK because you know deep down in your heart that at least your balls will live to see another day.
"Hey man take off your shirt and let me shoot you with my paint ball gun from across the yard."
"OK!" (walks across yard and turn around)
(friend FIRES quickly)
"WHAT THE FUCK MAN!! WHY'D YOU SHOOT I DIDN'T EVEN HAVE MY BALL CAGE UP YET!!!!"