This card's [When Digivolving] effect reads "You may place 1 card with [Royal Knight] in its traits and a play cost of 13 or less from your hand or trash under this Digimon as its bottom digivolution card. Activate 1 of that card's [When Digivolving] effects as an effect of this Digimon." At what timing does that [When Digivolving] effect activate?
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The effect is activated directly after placing the play cost 13 or lower [Royal Knight] trait card in its digivolution cards.The placed card's [When Digivolving] effect is resolved as a part of this effect, and once that is done, you may declare an attack with the <Blitz> from this effect.
Source: BT10-112 Jesmon GX