指切りげんまん

Yubikiri genman
Two people lock their pinkies when making a promise

More explanation here.

I think this lock pinkies thing has spreaded to the rest of Asia without having to sing the song. By locking the pinkies, the promise is officially "sealed" and to be remembered, that's all. I'm a bit shock to see the Japanese song of "drink 1000 needles and cut the finger" part...