Add a Bonus to an Action

Another use for a stunt is to give a skill an automatic bonus under a particular, very narrow circumstance, effectively letting a character specialize in something. The circumstance should be narrower than what the normal action allows, and only apply to one particular action or pair of actions.
The usual bonus is +2 to the skill total. However, if you want, you can also express the bonus as two shifts of additional effect after the roll succeeds, if that makes more sense. Remember, higher shifts on a roll allow your action to be more effective in certain ways.
You can also use this to establish any effect worth two shifts as an additional benefit of succeeding at the skill roll. This might be Fair (+2) passive opposition, the equivalent of a 2-point hit, a mild consequence, or an advantage that takes Fair (+2) opposition to remove. (Fate Core, p.90)