Description per SWN
Prime Attribute: Reflexes or Perception
Expertise: Choose an area of Expertise. This should be a broad category, such as Science, Engineering, Larceny, or whatever the player decides upon. When performing within their area of expertise, the Expert rolls the Expertise die in addition to their d20
Attack Bonus: At levels 3, 6, and 9, the Expert receives a +1 bonus to attack rolls.
Like a Charm: Once per session, the Expert may choose to automatically succeed on a skill attempt related to their Expertise.
(Expertise Die: Goodman Games upcoming Dungeon Crawl Classics is going to use an extra die for Warrior attacks that increases as the Warrior levels. I'm considering using this idea for all three classes, attacks for warrior, expertise for expert, and Psionic activation for Psionics.)