Scenario: It's night, and a man is walking home. He sees ahead there is a woman going in the opposite direction as him. Knowing not to stop or approach her, but his presence makes her nervous. What would help further in this scenario if anything?
I have always read it as down nod is for acknowledging someone you don't know/ know very well, and up nod is for people you know. The idea being you nod down to cover your vital neck while also making eye contact, in a sorta "I see/I am watching you if you try something". Up nod reveals the neck, showing trust and comfortability.