the Nod consists of a up and down motion, this motion is shared mainly by minority groups, or those seeking to be included in a minority group. Since that group, which is smaller then the greater ethnicity is in need of "coolness" and acceptance saying a greeting such as "hello" is not acceptable, instead the member of the group motions his head downwards, and the other upwards, to acknowledge their exsistance in the culture. Included in the 'nod' can also be a quick "eh" or "hows it goin", anymore communication then that is unacceptable due to the lack of relationship between the two members.
Guy 1: *nod*, "Sup"
Guy 2: *nod*, "eh, hows it going"