When you buy a new pack of cigarettes, you take two out and flip them upside down, and smoke those two last. It’s “one for luck, and one for fuck” — one for good luck, and one to smoke after you’ve had sex, often to share with the person.
This is a common tradition among many smokers, especially in New York. My informant first heard it in early high school, from girls that would smoke between classes. She’s been doing it as long as she’s been smoking.
Since smoking is generally considered a vice, I think it’s probably a way to ‘negate’ that, and make it something ‘lucky’ and good.