If the Attack is successful then after resolving a Damage Roll, deduct 2 from the Success Level of the Attack.
If the new Success Level is 0 or greater make another Damage Roll against the Defender.
Repeat this process until the Success Level is less than 0.
If the Success Level of the original attack was 0 or 1, after resolving a Damage Roll, make one additional Damage Roll against the Defender at Success Level 0.
NOTE
If a model has abilities that trigger when damage is caused each damage roll will trigger those abilities separately.