You roll first Mind stat roll (+ add any ranks from attributes which enchance your detection/senses) against the stealth roll (by rolling 2d6). If you beat the result, you see attack coming and can roll defense. If you fail though, then you do not roll defense at all against the attack and suffer consequences. In this case the air cannon does flat 10pts non-lethal dmg (minus any armor attribute effects) and causes knockback for 3 yards from impact point.