From Project: Redcap
Puissant Ability is a Minor General Virtue that makes the character especially adept with any one Ability (chosen when the character is created). It grants the character a flat bonus while using that Ability.
Puissant Ability increases the effective Ability score directly, whereas Affinity with Ability increases the experience points in the Ability. At low scores (up to level 8), Puissant Ability is more advantageous. From level 9 (225 experience points before multiplication), Affinity provides a higher effective score. In in-game terms, Puissant Ability represents a natural competency, a natural knack for the Ability; Affinity with Ability represents a capability to gain more from sources, to quickly pick up and improve the Ability, perhaps borne out of natural skill or perhaps from intense interest and focus.
There are errata for the text of this Virtue:
Replace the first sentence of the text with: "You are particularly adept with one Ability, and add 2 to its value whenever you use it. Note that you do not, in general, use an Ability when learning it, teaching it, or writing about it."
Therefore, Puissant Magic Theory would allow a magus to use more vis than usual in the lab, but Puissant Ability would not affect teaching, reading, or writing.
Puissant Ability does not actually change the character's Ability score, so it has no effect on the number of Experience Points needed to improve the Ability. For example, a character with a score of 5 in a given Ability, and the Puissant Ability Virtue, would need (in ArM5) 25 Experience Points to improve his Ability score to 6 (the same as would a character without Puissant Ability).
Puissant Ability does not change the maximum starting Ability score for a new character (p. 31), so (for example) a 22-year-old character would still have a maximum starting Ability score of 5, and then a bonus added on top of that score.
The history of this page before August 6, 2010 is archived at Legacy:puissant_ability