A Digimon with this card in its digivolution cards attacks, and another effect trashes a card from my hand. At such times, if I activate this card's inherited effect and perform a digivolution into P-209 [Titamon], can I then activate P-209 [Titamon]'s <Alliance>?
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No, you can't. <Alliance> is an effect that triggers upon an attack. You can't activate it if the Digimon doesn't have <Alliance> upon the attack declaration.
Source: BT24-021 SnowGoblimon