It replaces a die, before the dice pool is rolled. Only the wild die can "explode" (re-roll on a 6, and add the result to the first roll). The wild die also has the penalty of- when a 1 is rolled on the wild die- taking the highest normal die away, with an extra story problem as well. From the book you linked to me, page 6: Wild Die Whenever any you or GM, makes any roll, one of the dice must be different from the rest (in size or color). Designated as the Wild Die, this odd die represents the vagaries of life (like the direction of the wind affecting the flight of a bullet) that are too small to warrant their own difficulty modifiers. If you roll a 6 on the Wild Die, it is called a Critical Success and you may add the 6 to your total and roll the Wild Die again. As long as you roll Critical Successes on that die, you may continue to add them to your total and continue to roll. If you roll anything other than a 6, you add that number to the total and stop rolling. If you roll a 1 on the initial toss of the Wild Die, this is called a Critical Failure. The Critical Failure cancels out the highest roll. Then you add the remaining values, and the roll is determined normally, but a complication occurs. The GM gauges the significance of the complication by the total generated from a funny, "nearly didn't do it" result for a high total to a serious, "we have a problem" obstacle for a low total. When you roll a critical failure when shooting range weapons they overheat, for more information check Equipment chapter.
