Until the End Phase, all effects that add or subtract ATK or DEF are reversed. (Additions now subtract, and subtractions now add, instead. Multiplications and divisions, including halving/doubling, are not affected.)
Why can't this be activated in the damage step? The card affects damage calculations and point values. I can't see why it wouldn't be activated in the damage step also.
Wall of Disruption can only be activated on attack declaration, Reverse trap can be activated in battle step which comes right after attack declaration.
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