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.

I had seen the movie was out, but hadn’t gotten around to watching it. Glad to know it doesn’t actually exist, thank you.

It kinda sounds like they are intentionally putting squash in everything just for the opportunity to pick on you.