What is Back Party?
A sideshow party.
When one of your friends organize a party, you can boycott it and organize a Back Party to tease the friend or to show that people love you more. Indeed, a good BP will attract people from the original one (that had been advertised long time ago, contrary to your BP).
You also can recruit BP-men while in the original party. This must be done soberly and tactfully.
No party can survive a Festimano's back party.
No back party can survive a Festimano's party.