User blog comment:Starchythepotato/Oulu, Altri and Will Awakened! (7★)/@comment-112.198.64.8-20150529200522/@comment-25209037-20150529202728

Its not an ATK buff, but BB/SBB/UBB damage modifier boost. Each unit have a fixed damage modifier on their BB/SBB, for example Shida has a 400% damage modifier on his BB, when Oulu w/GC uses either his BB/SBB, this modifier is increased to 500%.