Can I trash just 1 face-down card from under a Tamer for the conditions of this card's [On Play] [When Digivolving] [When Attacking] effect?
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Official Ruling
No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this [On Play] [When Digivolving] effect require you to trash the specified number of cards under your Tamer.
Source: ST24-10 Lilamon