Using an affinity always requires the player to narrate an appropriate manifestation of the affinity. (A fireball for Ignis affinity, mind control for Digero, flight for Aeris, and so on).
When using an affinity for minor gain, the character will never get fewer successes than her current number of fixation dice. In short, a minor use sets the minimum number of successes generated by the roll. Thus, if a character has two fixation dice, but only rolls a degree of zero or one, her degree of success is automatically considered to be two.
The effect of a minor use gets particularly potent at three or more fixation dice, where the character is regularly able to create very powerful effects without any trouble at all.
When using an affinity for major gain, the character increases the degree of success by the number of fixation dice she has, in addition to rolling those dice. Major uses of an affinity require that the player voluntarily increase her character’s fixation by one as part of the roll.