Bro you know: nod the head up like “what up bro”

Bro you don’t know: nod the head down to show respect.

Supposedly, these types of greetings are ingrained from centuries ago in the sword and shield days, where if you didn’t know someone, you would give a downward head nod to not expose your neck (to get sliced).

Whereas if you knew the person you were greeting, you could give the upward nod and not worry about exposing your neck.