Meeting someone new is exciting, but sometimes you say goodbye without having any contact with that person and kick yourself. That's when you need to remember to "Pull a Francesca". When you're talking to a boy you just meet and you want to get to know them more, you would "Pull a Francesca" and ask for their number, give them yours or say something brave. Anything along the lines of that.
Boy: Hey nice meeting you, but I have to go, my rides here.
Girl: Oh okay! Bye!
Friend: *whispers to girl to start "Pulling a Francesca"*
Girl: Wait, what's your number?
Boy: *gives digits*