I would handle it by making it an affinity with the following rule. The affinity is not under the conscious control of the character but rather the player. When using the ability, the player states a slightly contrived set of circumstances that he would like to see happen (eg "When the Great Troll prepares to swing at me with his pole axe, his back gives out and he spends the rest of the round screaming in pain") and then use his luck affinity against the appropriate difficulty to see if it comes to pass. The only action the character would be doing is exclaiming "Wow, that was lucky" or words to that effect.
--Peter Metcalfe
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