Shield dice are rolled after you are targeted but before that enemy unit rolls attack dice. You must be in defensive form (with the exception of Parry dice) to roll shield dice. If you did not end your turn in defensive form, take 1 overfatigue to switch into shield form. If you have a shield item in your ready slot you do not need to take fatigue to switch battle form and you block 1 additional damage (without gaining fatigue). Then, take 1 light fatigue for each damage blocked by a rolled Shield die. If a Shield die would exhaust, instead return it to your player mat. Shield dice do not block true damage.