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.)
Rulings
Individual Card FAQs
If a card gives a monster +100 ATK, and "Reverse Trap" is activated, that monster now has -100 ATK instead. This card only affects increases and decreases of a defined value, such as +200 (which becomes -200) or -500 (which becomes +500). It does not change cards which "double", "half", or "reduce to 0" the ATK or DEF of a monster.
You cannot activate this card during the Damage Step. This card affects monsters, Spell Cards, and Trap Cards controlled by both players.
Additional "Reverse Traps" activated in the same turn will have no effect. So "Reverse Trap" will change "Axe of Despair'"s effect of adding 1000 ATK to subtracting 1000 ATK. If a second "Reverse Trap" is activated, the equipped monster still has -1000 ATK.
Netrep Rulings
If an effect sets ATK or DEF to a specific value, "Reverse Trap" will have no effect on this.
Trap|
Normal