If a street nigger pulled his malt liquor bottle away from his bumper lips and broke it on one of those nigger benches and then stabbed some caucasian people, that would be a good example of someone being nigger knifed.
To take a real life object like a pen, comb, stick, spoon, or anything and throw it like a throwing knife in MW2. Before throwing or when releasing the object, a person will usually say "Throwing knife!" to let them know what they were trying to do.