:When this unit boosts () a Kagero vanguard, if the number of rear-guards your opponent has is two or less, the boosted () unit gets +4000 until end of that battle.
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Official Q&A (4)
Q703 ·
Q. During an attack that this card boosts, the number of rear-guards on my opponent's field became 2 or less after using "Intercept". Does the ability of this card activate?
A. No it does not. As "When this card "Boosts"" is the timing when "Choose a unit that is to "Boost", and that card becomes "Rest"", and the timing to "Intercept" is after that timing, the ability of this card does not activate.
Q318 ·
Q. For units with "When this unit's attack hits", can I pay the cost twice to activate the ability twice when my unit's attack hits?
A. No, you cannot. [AUTO] abilities can only be activated once when the conditions are met(e.g. "When this unit~"). Cost can only be paid once as well.
Q301 ·
Q. If this unit is in the rear-guard and I perform its "Soul-Blast", do I pay the cost from the soul under the vanguard?
A. Yes. As rear-guards do not have soul beneath them, if the ability is activated from a unit in the rear-guard circle, the cards under the vanguard are moved to the drop zone.
Q124 ·
Q. When this card attacks, my opponent used intercept, and now has 2 or less rear-guards on the field. Does this card's ability activate?
A. No, it does not activate. "When this unit boosts" means "to choose a unit and put it to rest position", therefore intercept timing is already way past the ability's activation timing.