During the Damage Step, while an opponent's monster is attacking a monster you control: Target 1 other face-up monster you control; your attacked monster gains ATK equal to that target's ATK during the Damage Step only.
Basically this card serves almost the same purpose as cards that boosts attack damage. Just be mindful that this card cannot be activated during damage calculation. It can be activated from the start of the Damage Step until before damage calculation.
Your monsters gain ATK
It's like "Gift of the martyr" without sending your Monster to GY