callidus: [Latin: experienced, clever, dexterous, skillful.]
System: Use of Callidus requires spending one Willpower point. Each level in Callidus adds directly on to Dexterity for nearly any conceivable roll. In addition, the difficulty # for the action is lowered by one for every Callidus Level.
For example: Migel fires his pistol at Sean. Migel spends a Willpower point on the attack, and he has level-2 Callidus (Callidus • •). Migel rolls his regular attack dice, plus 2 additional dice for his Callidus. The difficulty is normally 6 but now it's 4 because he has level-2 Callidus. Finally he gets to add 1 automatic success to his roll (for the WP point he spent).
Those who use Callidus whilst employing Celerity gain the benefit of the extra dice for one action only. Extra WP points will allow the character extra dice on subsequent actions within that round. Furthermore, the difficulty number can never be reduced below 3.
In short, if you want to spend a Willpower point to ensure success in a single dex-related action, Callidus simply adds extra dice to the roll and lowers the difficulty of the action.