Rulings
Q1: Is the "while you have no face-up security cards" condition met when I have 0 cards in my security stack?
Yes, it is.
Q2: Can I also activate this card’s Main effect when I have 0 cards in my security stack?
Yes, you can. In such cases, you can't add a card from your security stack to your hand, therefore you only place this card in your security stack.
Q3: What happens to cards placed face up in the security stack by effects?
They become face-up security cards that remain revealed. Other than rules that specify face-up security cards, the rules apply in the same manner as standard security cards.
Q4: What happens upon a security check for a security card that is placed face-up?
The check is performed with the card left revealed. Other than rules for cards left revealed, the rules apply in the same manner as standard security checks.
Q5: Does a card's Security effect trigger upon a security check with that card placed face-up?
Yes, it triggers.
Q6: What happens if I shuffle a security stack that includes security cards placed face-up?
Any face-up cards are placed face down, then you shuffle the cards. After shuffling, all cards are left face-down.
Data Sources
Wind Guardians BT21-095 C
WG
While you have no face-up security cards, you can ignore this card's color requirements.
{Security} [Your Turn] All of your Digimon with the [WG] trait gain <Vortex>.
[Main] Add your bottom security card to the hand. Then, place this card face up as the bottom security card.
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[Security] You may play 1 [WG] trait level 5 or lower Digimon card from your hand without paying the cost.Wind Guardians BT21-095 C
WG
While you have no face-up security cards, you can ignore this card's color requirements.
{Security} [Your Turn] All of your Digimon with the [WG] trait gain <Vortex>.
[Main] Add your bottom security card to the hand. Then, place this card face up as the bottom security card.
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[Security] You may play 1 [WG] trait level 5 or lower Digimon card from your hand without paying the cost.Wind Guardians BT21-095 C WG While you have no face-up security cards, you may ignore this card's color requirements. (Security) [All Turns] All of your [WG] trait Digimon gain <Vortex>. [Main] Add your bottom security card to the hand. Then, place this card face up as the bottom security card. ---