Description from Core Rules: The character is familiar with non-weapon uses of gunpowder as signal flames, decorative fireworks and small explosives, as well as the safe handling of gunpowder and gunpowder recipes. If the character invests an extra slot in the proficiency, he is a master fireworks artist, capable of creating huge fuse-based displays for parties and special events.
This non-weapon proficiency takes 1 slot for Rogues or Wizards, two for other classes. Its skill check is INT with a penalty of -3. When coupled with magical fireworks spells, the caster acts as if 2 levels higher (although he may-not cast spells higher than those he can cast normally or cast more of them than his level/bonuses allow) Illusionists with Fireworks can create very believable firework illusions, which are -3 to disbelieve.