It is declared at the start of the player's initiative, and if the roll is successful, he gains a +1 (or even a +2) to his attack roll.
Here is the 'book' description:
Dirty Fighting: Veteran brawlers and soldiers acquire a repertoire of feints, ruses, and various unsportsmanlike tactics that can come in handy in a fight. A character with this "skill" can attempt to use a dirty trick once per fight; if he succeeds, he gains a +1 bonus to his next attack roll. If there's some reason the enemy believes the character will fight honorably (hardly a wise assumption!) the bonus is +2. Once a particular enemy has fallen prey to the character's dirty trick, he can never be caught off-guard again. In addition, if the character's opponent is skilled in dirty fighting himself, the attempt automatically fails.
Dirty Fighting costs one slot for Rogues and Men-at-Arms, and the score is equivalent to Intelligence with no bonuses.