Bob attacks with a Craw Wurm (6/4). Sue blocks it with two Durkwood Boars (4/4). After declaring blockers, she uses Helm of Chatzuk to give banding to one of the Boars. Because one of the Boars has banding, Sue--not Bob--decides how damage is distributed to them during damage dealing. The fact that neither one had banding when blockers were declared is irrelevant. Though Sue can pile all of the damage onto one of her Boars, she instead assigns 3 damage to each of them so that neither one dies; Bob's Wurm takes 8 damage from the Boars and dies.