I used this card's [Start of Your Main Phase] effect to play a card with an [On Play] effect. At such times, can I activate the [On Play] effect on the played card before processing the part of this card's [When Digivolving] effect after "then"?
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No, you can't. Once all of the processing is resolved for this card's [When Digivolving] effect, then you can activate the [On Play] effect on the played card.
Source: BT22-042 Nyabootmon