Zafo is typically heard in a situation in which a group of friends are deciding who is going to do a chore (e.g. getting ice from the convenience store, opening the door when somebody is knocking and everybody is slacking, calling a pizza delivery place). Someone asks "who is going to do this?" and each of the present answers ¡zafo! It is agreed that the last person replying has the obligation to do the chore. When in Mexico, politely obey this unwritten rule.
(Family watching a soccer match)
Abuelo: ¿Quién va a cambiarle el pañal a Manuelito?
Papá: ¡Zafo!... ¡Chale! Está bien. Yo lo hago, pero al medio tiempo.
Grandfather: Who's gonna change Manuelito's diaper?
Grandmother: Count me out!
Mom: Not me!
Little brother: I'm out!
Aunt: I'm gonzo!
Dad: Not me!... Fuck! I'll do it. But let's wait until halftime.
Changing diapers can be a real drag. Practice your zafos before having children.