Knockback, in our campaign, has two definitions: as a side effect of physical combat, and “on purpose”. Certain Powers can prevent, or to some measure negate, Knockback in either case.
Every attack that does Physical damage has the potential to knock its target backward. The distance (in APs) that the target is Knocked Back by an attack is equal to the number of Column Shifts that the attacker rolled on the Action Table while resolving the attack, minus the target’s weight in APs1. The maximum distance that a target may be Knocked Back by an attack equals the Effect Value of the attack minus the target’s weight in APs.
When a target is Knocked Back, it travels the appropriate distance in a straight line away from the attacker. If the target’s path sends it reeling into an obstacle or another Character, both the target and the obstacle (or Character) immediately receive a Physical Attack with an AV/EV equal to the distance that the target was Knocked Back. Any Character or obstacle attacked in this fashion, including the original target, defends with an OV/RV equal to its Body/Body. Hero points may be spent to increase the OV/RV if a character is involved, but the AV/EV of the Knockback cannot be changed, nor may Hero Points be used to add to the OV/RV of an inanimate object. Characters take normal Physical damage from such attacks, while obstacles may break.
When a combatant chooses to do Knockback “on purpose”, the attack does no direct damage, regardless of the form it takes. Instead, all RAPs on the attack roll (minus the target’s weight in APs) are converted into Knockback distance; if the Knockback distance equals or exceeds the APs of the target’s Body, Mind, or Spirit Attributes, that target is Stunned.
Knockback causes lethal damage if the target is hurled into something capable of impaling or cutting, such as a glass window or a set of spikes. A fall can also cause lethal damage if it continues for too great a height; see the Climbing rules.
1Most human adult characters weigh around 2 APs.