You make a melee weapon Strike against a target using the required weapon. If you Hit or Critically Hit, you gain a +2 Bonus to your damage roll, and can make a second attack roll with all the Bonuses and Penalties as the first, targeting the creature’s Fortitude DC. If your second attack roll Hits, it does not deal any damage; instead, the target is Stunned 1, or Stunned 2 if both of your attack rolls were Critical Hits.
This second attack roll is an effect channeled through your weapon Strike, rather than being a separate Strike or attack, and does not trigger Reactions or effects separate from your weapon Strike.