TCG Rulings

• The effect to destroy a face-down Defense Position does not target. ^[1]

• It activates immediately at the start of the Damage Step. ^[1]

• If you attack a face-up monster and then activated "Book of Moon", if the attack target monster is in face-down Defense Position at the start of the Damage Step you can use this card’s effect to destroy it. ^[1]

References

1. ↑ ^{1.0 1.1 1.2} Konami Gameplay FAQ: Storm of Ragnarok -- Card Rulings (Version 1.0)