As differentiated from the Todd, a scary guy I went to high school with.
The nod is that inexplicable head twitch that boys seem to do in lieu of saying hello. It seems to be used predominantly as an acknowlegement: I see you but I don't know you well enough to say hello. Variations: We slept together but I don't know you well enough to say hello; We know each other well but I'd prefer not to be associated with you; We are friends but you have pissed me off; And my favorite: We are friends but I am too cool to speak out loud.
The upward nod seems primarily to be the one used in blowing off, the downward for better acquaintances.
I've been noticing this strange behavior (less common in the South) for years and meaning to document all its subtleties. I'm reminded of it daily while walking around the lake--I can usually rack up 3-4 nods per walk.
In general, the nod and all too-cool-for-you behaviors piss me off, but this one is so pervasive that I've decided it must be some secret part of male culture that I must study and understand.