"Nigga Down!" is to be shouted when one of your bros and/or fellow pimps (preferably african-american male) is struck by one or more high velocity objects (i.e. a bullet). One exception to this is if they are struck by one of the following: a baseball bat, an axe, a bus, yo mamma's tahoe, or a flying ho.
Jamal: Yo Antoine, we got hoes flying everywhere take cover boy imma get ma heater
Antoine: *gets hit by a flying ho*
Jamal: Oh Shit! Nigga Down! Nigga Down!