If my opponent doesn't trash a card in their hand and I don't play a Digimon card from my trash for this card's [When Digivolving] [When Attacking] [Once Per Turn] effect, does that mean it doesn't count toward a use of [Once Per Turn]?
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Official Ruling
No, it counts toward a use of [Once Per Turn]. The "your opponent may trash 1 card in their hand" effect allows your opponent to choose whether or not they will discard a card in their hand. The effect will be considered to have activated when they choose whether or not to discard it, therefore it counts as a use of [Once Per Turn].
Source: EX8-063 Barbamon (X Antibody)