An opponent's Digimon with <Piercing> attacks my [CS] trait Digimon, then when my Digimon would be deleted in battle, I activate this card's [All Turns] effect and place my Digimon on top of the security stack. At such times, can <Piercing> on my opponent's attacking Digimon activate?
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No, it can't. <Piercing> triggers when a Digimon with that effect attacks and the opponent's Digimon is deleted in battle. Your [CS] trait Digimon that would be deleted is placed on top of the security stack by this card's [All Turns] effect, preventing that deletion. Therefore, <Piercing> on your opponent's Digimon doesn't trigger or activate.
Source: BT23-080 Yu Nogi