3 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

I have this Digimon in play, and attack with a black Digimon with 12000 DP, whose <When Attacking> effect grants itself +1000 DP, bringing its total DP up to 13000. Does this Digimon's effect allow that black Digimon to have <Piercing> during the battle?
Yes. This Digimon's effect becomes active as soon as your black Digimon reaches 13000 DP or more. Your black Digimon will conduct its battle with <Piercing>.
Ruled Mar 28, 2024
I attack with a black Digimon with 12000 DP, whose <When Attacking> effect grants itself +1000 DP, bringing its total DP up to 13000. If this Digimon is in its digivolution cards, does this Digimon's inherited effect allow the black Digimon to have <Security Attack +1> during its battle?
Yes. This Digimon's inherited effect becomes active as soon as your black Digimon reaches 13000 DP or more. Your black Digimon will conduct its battle with <Security Attack +1>.
Ruled Mar 28, 2024
This card is in the digivolution cards of a black Digimon with 13000 DP, which I use to attack my opponent. The first card flipped over during the security check causes the black Digimon to <De Digivolve>, resulting in it having 12000 DP or less. Does the black Digimon still get extra security checks from <Security Attack +1>?
This card's inherited effect is a <Your Turn> effect, so your black Digimon will lose <Security Attack +1> as soon as it drops below 13000 DP. If the Digimon has no other effects to increase the number of security checks, the attack will end without performing any further checks.
Ruled Mar 28, 2024
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Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.