A Piercing attack reduces the dice level of any Defense dice by the specified number. For instance, a Piercing 2 attack would reduce any Defense on the Reaction by two dice levels.
Unlike with normal dice level reductions, Defense dice affected by Piercing can be reduced below 1d2; if the dice level is reduced beyond 1d2, then no Defense dice are rolled. So, applying a Piercing 2 effect to a 1d4 Defense entry would eliminate the Defense dice altogether. Similarly, a Piercing 2 effect combined with another Defense dice level reduction of 1 or more would eliminate a 1d6 Defense entry.