When my opponent had 2 cards in their security attack, they activated [ST10-06 Mastemon]'s [When Digivolving] effect, and the effect that places a card from their trash in their security stack and the effect that plays a level 5 or lower Digimon card activated. My opponent temporarily has 3 cards in their security stack, then it becomes 2 cards. Can I activate this card's [All Turns] effect?
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Official Ruling
No, you can't activate it. Even if an effect activates and increases your opponent's security stack, their security stack must have 3 or more cards when the effect finishes activating.
Source: BT16-056 Publimon