Bujingi Sinyou

The monster effect "(1): From the start of the Damage Step until before damage calculation, when your Beast-Warrior-Type "Bujin" monster battles an opponent's monster: You can banish this card from your Graveyard; that monster you control gains ATK equal to the ATK of the opponent's monster it is battling until the end of the Damage Step, also any battle damage it inflicts to your opponent is halved" is a Quick Effect that activates in the Graveyard. (It does not target. Banishing "Bujingi Tsumugari" itself is the cost to activate this effect.)

You cannot activate the effect of "Bujingi Tsumugari" if the ATK of the opponent's monster you are battling is 0.

Both the process of increasing ATK and the process of halving the battle damage inflicted to your opponent are applied only during that Damage Step in which the effect of "Bujingi Tsumugari" was activated. (Once the attack ends and the game returns to the Battle Step, the effect is no longer applied.)

The ATK gained is equal to the ATK value of the opponent's monster currently applied on the field. (It is not equal to the original ATK, so if the opponent's monster's ATK has been changed by the effect of an Equip Spell card or similar, it will gain ATK equal to that changed ATK value.)