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?