Does anyone know the effect of this quality, it appears 4 times in the core rules and I don’t find its effect
regards,
Those 4 occurances are amongst other, similar damage quality lists, where not “Knockback” but “Knockdown” is listed.
“Knockback” must be an oversight in replacing Knockback by Knockdown, the term that is used throughout the book (with those 4 exceptions).
And the Knockdown quality is defined as such:
If one or more Dark Symmetry Icons are generated on the damage
roll, the target is knocked prone unless it can pass an Athletics
test with a difficulty equal to the number of Dark Symmetry Icons
generated.
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you should be right, as twice occurences are about the Punisher which has the knockdown quality
In many 2d20 products you find such confusing terms which are some remnants of some changes in the actual terms used in the respective rules books.
Of course, nothing that couldn’t be found and corrected by careful editing. 
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