Index -> Playing the Game -> Combat -> Combat Actions -> Playing a Tactic
Tactic cards can be played only during combat. They often have effects that improve your total attack or total defense in combat, though some have other effects. To play a tactic, double-click it in your hand when it is your turn to perform a combat action.
Tactics can only be played under specific conditions. Like activated actions, eligible tactics use the 
icon (if you're attacking) or the 
icon (if you're defending). Many tactics have both these icons.
If you meet all the requirements for playing a tactic, a hinted border will appear around it.
The effects of a tactic are immediate, and last only until the end of the combat. You'll see an oversized version of any tactic that's played appear on the screen. Then that tactic is discarded.
