A character that fails a Toughness save may be knocked back by the force of the impact.
To check if a character is knocked back the following conditions must be true:
The defending character must have been Stunned by the attack.
The attacking player must call for a knockback test.
The attack must be influcting blunt standard damage. Lethal, Penetrating, slashing and piercing damage may not inflict a knockback, unless they also have the Knockback Power Feat.

To check for knockback:

Roll 1D20+Knockback Resistance vs DC = 1Attack’s Damage Bonus

Knockback Resistance is equal to half of the target’s Toughness save bonus (round down). Impervious Toughness save bonus is not halved, subtract the full amount instead. Subtract the rank of the target’s Immovable power, if any (see page 89). Subtract 4 for each size category larger than Medium (a Huge character subtracts 8, for example). Add 1 for each size category smaller than Medium (a Small character adds 1, for example).

If the roll succeeds, there is no knockback.
If it fails, the target is knocked back.

To determine the distance of the knockback, find the amount by which the roll was missed on the value column of the Prograssion Table. That’s the distance (in feet) the character is knocked back.
A result of less than 5 feet means the character falls prone in roughly the same spot (although a short knockback distance may matter if the character is standing on a ledge, for example).
So a knockback result of 8, read on the Time and Value Progression Table, means the character is knocked back 250 feet.

You can always choose to inflict less than your attack’s maximum knockback (just like you can choose to do less than your attack’s maximum damage).


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