Heresypedia

Some weapons create a deafening blast or shock wave when they hit. When a target is struck by a weapon with the Concussive (X) quality, it must take a Toughness Test with a penalty equal to 10 times the number in parenthesis (X). For example a weapon with Concussive (2) would impose a –20 on Toughness Tests, and Concussive (0) would be a Test with no penalty. If he fails, target is stunned for 1 round per Degree of Failure. Additionally, if the target takes Damage greater than his Strength Bonus, the target is knocked Prone.