Similar to regular chivalry, except that the obstacle faced by the woman is created by her soon to be rescuer. It seems it may also mean that the rescuer, or "nigga", if you will, performs chivalric behavior in a ghetto or less than favorable manner.
The correct way to highlight a moment of true nigga chivalry is to follow the summary of the scenario or the line of dialogue with the sentence "Nigga chivalry."
He steals a girl's purse then "finds it". Nigga chivalry.
He lets the air out of her tires, then offers her a ride home. Nigga chivalry.
Or, she tells him she is pregnant, and he responds, "Shit girl, you said you was on the pill... Well, I'll be responsible... Let me buy you a drink." Nigga chivalry.